cifs: fix out-of-bound memory access when calling smb3_notify() at mount point

If smb3_notify() is called at mount point of CIFS, build_path_from_dentry()
returns the pointer to kmalloc-ed memory with terminating zero (this is
empty FileName to be passed to SMB2 CREATE request). This pointer is assigned
to the `path` variable.
Then `path + 1` (to skip first backslash symbol) is passed to
cifs_convert_path_to_utf16(). This is incorrect for empty path and causes
out-of-bound memory access.

Get rid of this "increase by one". cifs_convert_path_to_utf16() already
contains the check for leading backslash in the path.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=212693
CC: <stable@vger.kernel.org> # v5.6+
Signed-off-by: Eugene Korenevsky <ekorenevsky@astralinux.ru>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2ops.c

@@ -2264,7 +2264,7 @@ smb3_notify(const unsigned int xid, struct file *pfile,
 		goto notify_exit;
 	}
 
-	utf16_path = cifs_convert_path_to_utf16(path + 1, cifs_sb);
+	utf16_path = cifs_convert_path_to_utf16(path, cifs_sb);
 	if (utf16_path == NULL) {
 		rc = -ENOMEM;
 		goto notify_exit;