cifs: make cifs_ioctl handle NULL filp->private_data correctly

Commit 13cfb7334e made cifs_ioctl use the tlink attached to the
cifsFileInfo for a filp. This ignores the case of an open directory
however, which in CIFS can have a NULL private_data until a readdir
is done on it.

This patch re-adds the NULL pointer checks that were removed in commit
50ae28f01 and moves the setting of tcon and "caps" variables lower.

Long term, a better fix would be to establish a f_op->open routine for
directories that populates that field at open time, but that requires
some other changes to how readdir calls are handled.

Reported-by: Kjell Rune Skaaraas <[e-mail]>
Reviewed-and-Tested-by: Suresh Jayaraman <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/ioctl.c

···
 	struct cifs_sb_info *cifs_sb;
 #ifdef CONFIG_CIFS_POSIX
 	struct cifsFileInfo *pSMBFile = filep->private_data;
-	struct cifsTconInfo *tcon = tlink_tcon(pSMBFile->tlink);
+	struct cifsTconInfo *tcon;
 	__u64	ExtAttrBits = 0;
 	__u64	ExtAttrMask = 0;
-	__u64   caps = le64_to_cpu(tcon->fsUnixInfo.Capability);
+	__u64   caps;
 #endif /* CONFIG_CIFS_POSIX */
 
 	xid = GetXid();
···
 			break;
 #ifdef CONFIG_CIFS_POSIX
 		case FS_IOC_GETFLAGS:
+			if (pSMBFile == NULL)
+				break;
+			tcon = tlink_tcon(pSMBFile->tlink);
+			caps = le64_to_cpu(tcon->fsUnixInfo.Capability);
 			if (CIFS_UNIX_EXTATTR_CAP & caps) {
 				rc = CIFSGetExtAttr(xid, tcon, pSMBFile->netfid,
 					&ExtAttrBits, &ExtAttrMask);
···
 			break;
 
 		case FS_IOC_SETFLAGS:
+			if (pSMBFile == NULL)
+				break;
+			tcon = tlink_tcon(pSMBFile->tlink);
+			caps = le64_to_cpu(tcon->fsUnixInfo.Capability);
 			if (CIFS_UNIX_EXTATTR_CAP & caps) {
 				if (get_user(ExtAttrBits, (int __user *)arg)) {
 					rc = -EFAULT;