kernel/fs: Increment pos in readv/writev so long as it's not -1.

This amends 20ac27def6.

I missed an important case in that commit: if pos is not -1,
then movePosition will still be false, but we nonetheless
need to increment the read/write position.

Should fix #18921.

src/system/kernel/fs/fd.cpp

@@ -819,7 +819,7 @@ common_user_vector_io(int fd, off_t pos, const iovec* userVecs, size_t count,
 		else
 			bytesTransferred += (ssize_t)length;
 
-		if (movePosition)
+		if (pos != -1)
 			pos += length;
 
 		if (length < vecs[i].iov_len)
@@ -1095,7 +1095,7 @@ _kern_readv(int fd, off_t pos, const iovec* vecs, size_t count)
 		else
 			bytesRead += (ssize_t)length;
 
-		if (movePosition)
+		if (pos != -1)
 			pos += vecs[i].iov_len;
 	}
 
@@ -1188,7 +1188,7 @@ _kern_writev(int fd, off_t pos, const iovec* vecs, size_t count)
 		else
 			bytesWritten += (ssize_t)length;
 
-		if (movePosition)
+		if (pos != -1)
 			pos += vecs[i].iov_len;
 	}