ceph: fix comments mentioning i_mutex

inode->i_mutex has been replaced with inode->i_rwsem long ago. Fix
comments still mentioning i_mutex.

Signed-off-by: hongnanli <hongnan.li@linux.alibaba.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>