fscrypto: require write access to mount to set encryption policy

Since setting an encryption policy requires writing metadata to the
filesystem, it should be guarded by mnt_want_write/mnt_drop_write.
Otherwise, a user could cause a write to a frozen or readonly
filesystem.  This was handled correctly by f2fs but not by ext4.  Make
fscrypt_process_policy() handle it rather than relying on the filesystem
to get it right.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: stable@vger.kernel.org # 4.1+; check fs/{ext4,f2fs}
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/ext4/ioctl.c

@@ -776,7 +776,7 @@ resizefs_out:
 				   (struct fscrypt_policy __user *)arg,
 				   sizeof(policy)))
 			return -EFAULT;
-		return fscrypt_process_policy(inode, &policy);
+		return fscrypt_process_policy(filp, &policy);
 #else
 		return -EOPNOTSUPP;
 #endif