f2fs: Restore rwsem lockdep support

Lockdep uses lock class keys in its analysis. init_rwsem() instantiates
one lock class key with each init_rwsem() user as follows:

 #define init_rwsem(sem)                                        \
 do {                                                           \
         static struct lock_class_key __key;                    \
                                                                \
         __init_rwsem((sem), #sem, &__key);                     \
 } while (0)

Commit e4544b63

 ("f2fs: move f2fs to use reader-unfair rwsems") reduced
the number of lock class keys from one per init_rwsem() user to one per
file in which init_f2fs_rwsem() is used. This causes the same lock class key
to be associated with multiple f2fs rwsems and also triggers a number of
false positive lockdep deadlock reports. Fix this by again instantiating one
lock class key with each init_f2fs_rwsem() caller.

Cc: Tim Murray <timmurray@google.com>
Reported-by:  <syzbot+0b9cadf5fc45a98a5083@syzkaller.appspotmail.com>
Fixes: e4544b63

 ("f2fs: move f2fs to use reader-unfair rwsems")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>