gfs2: No longer use 'extern' in function declarations

For non-static function declarations, external linkage is implied and
the 'extern' keyword isn't needed.  Some static checkers complain about
the overuse of 'extern', so clean up all the function declarations.

In addition, remove 'extern' from the definition of
free_local_statfs_inodes(); it isn't needed there, either.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>