FreeBSD: Use unmapped I/O for scattered/gang ABD buffers

Many FreeBSD disk drivers support "unmapped" I/O mode, when data
buffer represented not with a virtually contiguous KVA-mapped address
range, but with a list of physical memory pages.  Originally it was
designed to do I/O from buffers without KVA mapping (unmapped).  But
moving virtual addresses out of equation allows us to operate even
non-contiguous data buffers with one condition: all buffer discon-
tinuities must be aligned to memory page borders.

Doing I/O to capable GEOM device this patch traverses through non-
linear ABD buffers, validating the chunks borders.  If the condition
is met, it supplies GEOM with the list of original physical memory
pages instead of copying the data into temporary contiguous buffer.
On capable hardware on pools with ashift=12 and default ABD chunk of
4KB it should handle all the I/O without additional memory copying.

Reviewed-by: Brian Atkinson <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #12320

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