xfs: add missing error check in xfs_prepare_shift()

xfs_prepare_shift() fails to check the error return from
xfs_flush_unmap_range(). If the latter fails, that could lead to an
insert/collapse range operation over a delalloc range, which is not
supported.

Add an error check and return appropriately. This is reproduced
rarely by generic/475.

Fixes: 7f9f71be ("xfs: extent shifting doesn't fully invalidate page cache")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>