fuse: fix READDIRPLUS skipping an entry

Marios Titas running a Haskell program noticed a problem with fuse's
readdirplus: when it is interrupted by a signal, it skips one directory
entry.

The reason is that fuse erronously updates ctx->pos after a failed
dir_emit().

The issue originates from the patch adding readdirplus support.

Reported-by: Jakob Unterwurzacher <[e-mail]>
Tested-by: Marios Titas <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>
Fixes: 0b05b18381ee ("fuse: implement NFS-like readdirplus support")
Cc: <[e-mail]> # v3.9

fs/fuse/dir.c

···
 			*/
 			over = !dir_emit(ctx, dirent->name, dirent->namelen,
 				       dirent->ino, dirent->type);
-			ctx->pos = dirent->off;
+			if (!over)
+				ctx->pos = dirent->off;
 		}
 
 		buf += reclen;