cifs: check ntwrk_buf_start for NULL before dereferencing it

RHBZ: 1021460

There is an issue where when multiple threads open/close the same directory
ntwrk_buf_start might end up being NULL, causing the call to smbCalcSize
later to oops with a NULL deref.

The real bug is why this happens and why this can become NULL for an
open cfile, which should not be allowed.
This patch tries to avoid a oops until the time when we fix the underlying
issue.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/readdir.c

@@ -655,7 +655,14 @@ find_cifs_entry(const unsigned int xid, struct cifs_tcon *tcon, loff_t pos,
 		/* scan and find it */
 		int i;
 		char *cur_ent;
-		char *end_of_smb = cfile->srch_inf.ntwrk_buf_start +
+		char *end_of_smb;
+
+		if (cfile->srch_inf.ntwrk_buf_start == NULL) {
+			cifs_dbg(VFS, "ntwrk_buf_start is NULL during readdir\n");
+			return -EIO;
+		}
+
+		end_of_smb = cfile->srch_inf.ntwrk_buf_start +
 			server->ops->calc_smb_size(
 					cfile->srch_inf.ntwrk_buf_start,
 					server);