xfs_io: factor out new get_last_inode() helper

xfs_io: factor out new get_last_inode() helper

The inode command by default finds the last allocated inode in the
filesystem via bulkstat, and this specific function is open-coded
after other cases are handled, leading to a fairly long inode_f
function and confusing code flow.

Clean it up by factoring it into a new function, more refactoring
will follow.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>