xfs: move extent zeroing to xfs_bmapi_allocate

xfs: move extent zeroing to xfs_bmapi_allocate

Source kernel commit: fd638f1de1f3f736ea4debb3582999ea95506e0a

Move the extent zeroing case there for the XFS_BMAPI_ZERO flag outside
the low-level allocator and into xfs_bmapi_allocate, where is still
is in transaction context, but outside the very lowlevel code where
it doesn't belong.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>