cifs: allow calling SMB2_xxx_free(NULL)

Change these free functions to allow passing NULL as the argument and
treat it as a no-op just like free(NULL) would.
Or, if rqst->rq_iov is NULL.

The second scenario could happen for smb2_queryfs() if the call
to SMB2_query_info_init() fails and we go to qfs_exit to clean up
and free all resources.
In that case we have not yet assigned rqst[2].rq_iov and thus
the rq_iov dereference in SMB2_close_free() will cause a NULL pointer
dereference.

Fixes:  1eb9fb52 ("cifs: create SMB2_open_init()/SMB2_open_free() helpers")
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2pdu.c

@@ -2243,10 +2243,12 @@ SMB2_open_free(struct smb_rqst *rqst)
 {
 	int i;
 
+	if (rqst && rqst->rq_iov) {
 		cifs_small_buf_release(rqst->rq_iov[0].iov_base);
 		for (i = 1; i < rqst->rq_nvec; i++)
 			if (rqst->rq_iov[i].iov_base != smb2_padding)
 				kfree(rqst->rq_iov[i].iov_base);
+	}
 }
 
 int
@@ -2536,6 +2538,7 @@ SMB2_close_init(struct cifs_tcon *tcon, struct smb_rqst *rqst,
 void
 SMB2_close_free(struct smb_rqst *rqst)
 {
+	if (rqst && rqst->rq_iov)
 		cifs_small_buf_release(rqst->rq_iov[0].iov_base); /* request */
 }
 
@@ -2689,6 +2692,7 @@ SMB2_query_info_init(struct cifs_tcon *tcon, struct smb_rqst *rqst,
 void
 SMB2_query_info_free(struct smb_rqst *rqst)
 {
+	if (rqst && rqst->rq_iov)
 		cifs_small_buf_release(rqst->rq_iov[0].iov_base); /* request */
 }
 
@@ -3803,6 +3807,7 @@ SMB2_set_info_init(struct cifs_tcon *tcon, struct smb_rqst *rqst,
 void
 SMB2_set_info_free(struct smb_rqst *rqst)
 {
+	if (rqst && rqst->rq_iov)
 		cifs_buf_release(rqst->rq_iov[0].iov_base); /* request */
 }