fs/jffs2: fix comments mentioning i_mutex

#include <linux/mutex.h>

struct jffs2_inode_info {

	/* We need an internal mutex similar to inode->i_mutex.

	/* We need an internal mutex similar to inode->i_rwsem.

	   Unfortunately, we can't used the existing one, because

	   either the GC would deadlock, or we'd have to release it

	   before letting GC proceed. Or we'd have to put ugliness

	   into the GC code so it didn't attempt to obtain the i_mutex

	   into the GC code so it didn't attempt to obtain the i_rwsem

	   for the inode(s) which are already locked */

	struct mutex sem;