smb3: rc uninitialized in one fallocate path

Clang detected a problem with rc possibly being unitialized
(when length is zero) in a recently added fallocate code path.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2ops.c

@@ -3617,7 +3617,8 @@ static int smb3_simple_fallocate_write_range(unsigned int xid,
 					     char *buf)
 {
 	struct cifs_io_parms io_parms = {0};
-	int rc, nbytes;
+	int nbytes;
+	int rc = 0;
 	struct kvec iov[2];
 
 	io_parms.netfid = cfile->fid.netfid;