f2fs: mutex can't be used by down_write_nest_lock()

f2fs_lock_all() calls down_write_nest_lock() to acquire a rw_sem and check
a mutex, but down_write_nest_lock() is designed for two rw_sem accoring to the
comment in include/linux/rwsem.h. And, other than f2fs, it is just called in
mm/mmap.c with two rwsem.

So, it looks it is used wrongly by f2fs. And, it causes the below compile
warning on -rt kernel too.

In file included from fs/f2fs/xattr.c:25:0:
fs/f2fs/f2fs.h: In function 'f2fs_lock_all':
fs/f2fs/f2fs.h:962:34: warning: passing argument 2 of 'down_write_nest_lock' from incompatible pointer type [-Wincompatible-pointer-types]
  f2fs_down_write(&sbi->cp_rwsem, &sbi->cp_mutex);
                                  ^
fs/f2fs/f2fs.h:27:55: note: in definition of macro 'f2fs_down_write'
 #define f2fs_down_write(x, y) down_write_nest_lock(x, y)
                                                       ^
In file included from include/linux/rwsem.h:22:0,
                 from fs/f2fs/xattr.c:21:
include/linux/rwsem_rt.h:138:20: note: expected 'struct rw_semaphore *' but argument is of type 'struct mutex *'
 static inline void down_write_nest_lock(struct rw_semaphore *sem,

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>