Perform a lockless read to test whether an entropy havesting fifo is full, avoiding the cost of mutex operations if it is.  We re-test once the mutex is acquired to make sure it's still true before doing the -modify-write part of the read-modify-write.  Note that due to the maximum fifo depth being pretty deep, this is unlikely to improve harvesting performance yet.
Perform a lockless read to test whether an entropy havesting fifo is
full, avoiding the cost of mutex operations if it is.  We re-test
once the mutex is acquired to make sure it's still true before doing
the -modify-write part of the read-modify-write.  Note that due to
the maximum fifo depth being pretty deep, this is unlikely to improve
harvesting performance yet.

Approved by:	markm

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase