MFC 324072: Add UMA_ALIGNOF().

This is a wrapper around _Alignof() that sets the alignment for a zone
to the alignment required by a given type. This allows the compiler to
determine the proper alignment rather than having the programmer try to
guess.

MFC r280957

Fix integer truncation bug in malloc(9)

A couple of internal functions used by malloc(9) and uma truncated
a size_t down to an int. This could cause any number of issues
(e.g. indefinite sleeps, memory corruption) if any kernel
subsystem tried to allocate 2GB or more through malloc. zfs would
attempt such an allocation when run on a system with 2TB or more
of RAM.

MFC r282690:
Call uma_reclaim() from the additional pagedaemon thread to reclaim kmem
arena address space.

Merge r261722, r261723, r261724, r261725 from head:
several minor improvements for UMA_ZPCPU_ZONE zones.

MFC r280957

Fix integer truncation bug in malloc(9)

A couple of internal functions used by malloc(9) and uma truncated
a size_t down to an int. This could cause any number of issues
(e.g. indefinite sleeps, memory corruption) if any kernel
subsystem tried to allocate 2GB or more through malloc. zfs would
attempt such an allocation when run on a system with 2TB or more
of RAM.

MFC r282690:
Call uma_reclaim() from the additional pagedaemon thread to reclaim kmem
arena address space.

Merge r261722, r261723, r261724, r261725 from head:
several minor improvements for UMA_ZPCPU_ZONE zones.