[CIFS] ACPI suspend oops

Wasn't able to reproduce a hard hang, but was able to get an oops if
suspended the machine during a copy to the cifs mount.  This led to some
things hanging, including a "sync".  Also got I/O errors when trying to
access the mount afterwards (even when didn't see the oops), and had
to unmount and remount in order to access the filesystem.

This patch fixed the oops.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/file.c

@@ -904,8 +904,10 @@ static ssize_t cifs_write(struct file *file, const char *write_data,
 				if (rc != 0)
 					break;
 			}
-			if(experimEnabled || (pTcon->ses->server->secMode & 
-			 (SECMODE_SIGN_REQUIRED | SECMODE_SIGN_ENABLED)) == 0) {
+			if(experimEnabled || (pTcon->ses->server &&
+				(pTcon->ses->server->secMode & 
+				(SECMODE_SIGN_REQUIRED | SECMODE_SIGN_ENABLED)
+				== 0))) {
 				struct kvec iov[2];
 				unsigned int len;