MFC r344855 (by jhb):

Drop "All rights reserved" from my copyright statements.

Reviewed by: rgrimes
Differential Revision: https://reviews.freebsd.org/D19485

MFC r339600:
Make sure returned value is checked and assert a valid refcount.
While at it fix a print: Unsigned types cannot be negative.

Reviewed by: kib, mjg
Differential revision: https://reviews.freebsd.org/D17616
Sponsored by: Mellanox Technologies

MFC r331666: ZFS vn_rele_async: catch up with the use of refcount(9) for the vnode use count

It's not sufficient nor required to use the vnode interlock when
checking if we are going to drop the last use count as the code in
vputx() uses refcount (atomic) operations for both checking and
decrementing the use code. Apply the same method to vn_rele_async().
While here, remove vn_rele_inactive(), a wrapper around vrele() that
didn't add any value.

Also, the change required making vfs_refcount_release_if_not_last()
public. I've made vfs_refcount_acquire_if_not_zero() public as well.
They are in sys/refcount.h now. While making the move I've dropped the
vfs_ prefix.

Sponsored by: Panzura

Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

MFC r320501:
Correct fences for sys/refcount.h.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Drop the 3rd clause from all 3 clause BSD licenses where I am the sole
holder to convert them to 2 clause BSD licenses.

MFC after: 1 week

Add assertion for refcount overflow.

Submitted by: Andrey Zonov <andrey zonov.org>
Reviewed by: kib

Remove debugging cruft.

pjd@'s r180759 was intended to revert r180755 due to ipfilter breakage,
but removed too much, breaking the build in other places instead. Now
that the ipfilter issue has been fixed (or hacked around), address the
second issue by restoring r180755, with one small change. I don't feel
comfortable using assert(3) in a header that will be included in userland
code that may or may not already have an assertion mechanism in place,
so KASSERT() evaluates to a no-op in the !_KERNEL case.

Back out this change, as it breaks the build and I don't have time for
a better fix today.

Check for negative reference count.

Reviewed by: des

Add a simple reference count API that is simply a thin wrapper API around
atomic operations on ints.

Reviewed by: arch@
MFC after: 1 week