cifs: return error on invalid value written to cifsFYI

RHBZ: 1539617

Check that, if it is not a boolean, the value the user tries
to write to /proc/fs/cifs/cifsFYI is valid and return an error
if not.
Signed-off-by: NRonnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>
Reported-by: NXiaoli Feng <xifeng@redhat.com>

fs/cifs/cifs_debug.c

@@ -584,6 +584,8 @@ static ssize_t cifsFYI_proc_write(struct file *file, const char __user *buffer,
 		cifsFYI = bv;
 	else if ((c[0] > '1') && (c[0] <= '9'))
 		cifsFYI = (int) (c[0] - '0'); /* see cifs_debug.h for meanings */
+	else
+		return -EINVAL;
 
 	return count;
 }