gfs2: Clean up waiting on glocks

The prepare_to_wait_on_glock and finish_wait_on_glock functions introduced in
commit 56a365be "gfs2: gfs2_glock_get: Wait on freeing glocks" are
better removed, resulting in cleaner code.

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