xfs: remove the xfs_iext_peek_prev_extent call in xfs_bmapi_allocate

Both callers of xfs_bmapi_allocate already initialize bma->prev, don't
redo that in xfs_bmapi_allocate.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>