xfs: support XFS_BMAPI_REMAP in xfs_bmap_del_extent_delay

The zone allocator wants to be able to remove a delalloc mapping in the
COW fork while keeping the block reservation.  To support that pass the
flags argument down to xfs_bmap_del_extent_delay and support the
XFS_BMAPI_REMAP flag to keep the reservation.

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