kmod: throttle kmod thread limit

If we reach the limit of modprobe_limit threads running the next
request_module() call will fail. The original reason for adding
a kill was to do away with possible issues with in old circumstances
which would create a recursive series of request_module() calls.

We can do better than just be super aggressive and reject calls
once we've reached the limit by simply making pending callers wait
until the threshold has been reduced, and then throttling them in,
one by one.

This throttling enables requests over the kmod concurrent limit to
be processed once a pending request completes. Only the first item
queued up to wait is woken up. The assumption here is once a task
is woken it will have no other option to also kick the queue to check
if there are more pending tasks -- regardless of whether or not it
was successful.

By throttling and processing only max kmod concurrent tasks we ensure
we avoid unexpected fatal request_module() calls, and we keep memory
consumption on module loading to a minimum.

With x86_64 qemu, with 4 cores, 4 GiB of RAM it takes the following run
time to run both tests:

time ./kmod.sh -t 0008
real    0m12.364s
user    0m0.704s
sys     0m5.373s

time ./kmod.sh -t 0009
real    0m47.638s
user    0m1.033s
sys     0m5.425s

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>