xfs: return if_data from xfs_idata_realloc

Many of the xfs_idata_realloc callers need to set a local pointer to the
just reallocated if_data memory.  Return the pointer to simplify them a
bit and use the opportunity to re-use krealloc for freeing if_data if the
size hits 0.

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