libroot/unistd: Fix write truncating return values of _kern_write.

ssize_t is 64-bit on 64-bit architectures, if we truncate to int
then values above 2GB will be seen as negative and errors instead
of the successes they are.

Seems to fix #16861.

src/system/libroot/posix/unistd/write.c

@@ -21,7 +21,7 @@
 ssize_t
 write(int fd, void const *buffer, size_t bufferSize)
 {
-	int status = _kern_write(fd, -1, buffer, bufferSize);
+	ssize_t status = _kern_write(fd, -1, buffer, bufferSize);
 
 	RETURN_AND_SET_ERRNO_TEST_CANCEL(status);
 }