xfs: create individual inode alloc. helper

Inode allocation from sparse inode records must filter the ir_free mask
against ir_holemask.  In preparation for this requirement, create a
helper to allocate an individual inode from an inode record.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>