xfs: rework splitting of indirect block reservations

Move the check if we have enough indirect blocks and the stealing of
the deleted extent blocks out of xfs_bmap_split_indlen and into the
caller to prepare for handling delayed allocation of RT extents that
can't easily be stolen.

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