xfs: set xefi_discard when creating a deferred agfl free log intent item

xfs: set xefi_discard when creating a deferred agfl free log intent item

Source kernel commit: 2c334e12f957cd8c6bb66b4aa3f79848b7c33cab

Make sure that we actually initialize xefi_discard when we're scheduling
a deferred free of an AGFL block.  This was (eventually) found by the
UBSAN while I was banging on realtime rmap problems, but it exists in
the upstream codebase.  While we're at it, rearrange the structure to
reduce the struct size from 64 to 56 bytes.

Fixes: fcb762f5de2e ("xfs: add bmapi nodiscard flag")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>