Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

MFC r260581: Add sglist_append_bio(9) to append a struct bio's data to
a sglist

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

This patch fixes two bugs in sglist(9) and improves robustness of the API via
better semantics if a request to append an address range to an existing list
fails.
- When cloning an sglist, properly set the length in the new sglist instead of
leaving the new list empty.
- Properly compute the amount of data added to an sglist via
_sglist_append_buf(). This allows sglist_consume_uio() to properly update
uio_resid.
- When a request to append an address range to a scatter/gather list fails,
restore the sglist to the state it had at the start of the function call
instead of resetting it to an empty list.

Requested by: np (3)
Approved by: re (kib)

Change the 'resid' parameter to sglist_consume_uio() from an int to a
size_t to match the recent type change of the uio_resid member of struct
uio.

Approved by: re (kib)

Add a simple API to manage scatter/gather lists of phyiscal addresses.
Each list describes a logical memory object that is backed by one or more
physical address ranges. To minimize locking, the sglist objects
themselves are immutable once they are shared.

These objects may be used in the future to facilitate I/O requests using
physically-addressed buffers. For the immediate future I plan to use them
to implement a new type of VM object and pager.

Reviewed by: jeff, scottl
MFC after: 1 month