uloop: fix corner cases with recursive uloop_run calls

uloop: fix corner cases with recursive uloop_run calls

With multiple recursive calls to uloop_run, the callback for the same fd
can be run multiple times from different levels in the stack.
Prevent this by tracking the stack of uloop_fd callbacks and buffering new
incoming events for fds already on the stack.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>