eCryptfs: Unlock page in write_begin error path

Unlock the page in error path of ecryptfs_write_begin(). This may
happen, for example, if decryption fails while bring the page
up-to-date.

Cc: <stable@kernel.org>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>

fs/ecryptfs/mmap.c

@@ -381,6 +381,11 @@ static int ecryptfs_write_begin(struct file *file,
 	    && (pos != 0))
 		zero_user(page, 0, PAGE_CACHE_SIZE);
 out:
+	if (unlikely(rc)) {
+		unlock_page(page);
+		page_cache_release(page);
+		*pagep = NULL;
+	}
 	return rc;
 }