vfs: add minimal implementation of unlinkat

The aarch64 version of Linux does not implement SYS_unlink.
The musl stdio/tmpfile.c in such case falls back to SYS_unlinkat.

This patch adds minimal implementation of unlinkat just enough
to make tmpfile() functional. The tmpfile() calls unlinkat()
with dirfd = AT_FDCWD and flags = 0 which is what our version
of unlinkat() implements for now.

Signed-off-by: Waldemar Kozaczuk <jwkozaczuk@gmail.com>
Message-Id: <20201219210335.3003-1-jwkozaczuk@gmail.com>

fs/vfs/main.cc

@@ -1109,6 +1109,15 @@ int unlink(const char *pathname)
     return -1;
 }
 
+int unlinkat(int dirfd, const char *pathname, int flags)
+{
+    //TODO: Really implement it
+    if (dirfd != AT_FDCWD || flags) {
+        UNIMPLEMENTED("unlinkat() with non-zero flags or dirfd != AT_FDCWD");
+    }
+    return unlink(pathname);
+}
+
 TRACEPOINT(trace_vfs_stat, "\"%s\" %p", const char*, struct stat*);
 TRACEPOINT(trace_vfs_stat_ret, "");
 TRACEPOINT(trace_vfs_stat_err, "%d", int);