xfs_io: move inode command arg handling to top

xfs_io: move inode command arg handling to top

As it stands, collecting the inode number and testing args validity
is all tangled up; for example the test for "-n" having no inode is
buried in an else after a large code block which handles something
else.

Get inode number argument collection and testing out of the way
before doing anything else.

Clean up the error message if a non-numeric inode arg is given.

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