replace mutexes to serialise the operations on the flag that restricts the number of contexts that are refilling the rx rings with atomic ops.
replace mutexes to serialise the operations on the flag that restricts
the number of contexts that are refilling the rx rings with atomic
ops.

this is borrowed from code i wrote for the scsi midlayer but cant
put in yet because i havent got atomic.h up to scrach on all archs
yet. the archs myx runs on do have enough atomic.h to be fine though.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars