make VOL_STATE_VLRU_ADD exclusive

make VOL_STATE_VLRU_ADD exclusive

VLRU_Add_r() places a volume into VOL_STATE_VLRU_ADD before
calling VLRU_Wait_r(), which drops VOL_LOCK while waiting
for the required VLRU queue to quiesce.  Thus, it is essential
that state VOL_STATE_VLRU_ADD be marked exclusive so that
another thread cannot mutate state until the VLRU generation
add transaction completes.

Reviewed-on: http://gerrit.openafs.org/554
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>