xfs: insert newly allocated inode chunks into the finobt

A newly allocated inode chunk, by definition, has at least one
free inode, so a record is always inserted into the finobt.

Create the xfs_inobt_insert() helper from existing code to insert
a record in an inobt based on the provided BTNUM. Update
xfs_ialloc_ag_alloc() to invoke the helper for the existing
XFS_BTNUM_INO tree and XFS_BTNUM_FINO tree, if enabled.

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