modules/kvs: Fix forced dirty bit clear error

[chu11]

The function cache_entry_force_clear_dirty() should also destroy the
dirty waitqueue, as the entries in the waitqueue no longer matter
and it would prevent the cache entry from ever being reclaimed.

[codecov-io]

Codecov Report

Merging #1133 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1133      +/-   ##
==========================================
- Coverage   77.38%   77.33%   -0.06%     
==========================================
  Files         184      184              
  Lines       31886    31887       +1     
==========================================
- Hits        24675    24659      -16     
- Misses       7211     7228      +17

Impacted Files	Coverage Δ
src/modules/kvs/cache.c	91.22% <100%> (+0.15%)	⬆️
t/mpi/hello.c	93.54% <0%> (-6.46%)	⬇️
src/common/libflux/ev_flux.c	97.56% <0%> (-2.44%)	⬇️
src/common/libflux/mrpc.c	85.15% <0%> (-1.18%)	⬇️
src/modules/kvs/kvs.c	62.67% <0%> (-0.79%)	⬇️
src/connectors/local/local.c	86.82% <0%> (-0.78%)	⬇️
src/common/libkvs/kvs.c	64.58% <0%> (-0.76%)	⬇️
src/common/libflux/dispatch.c	85.18% <0%> (-0.75%)	⬇️
src/common/libkvs/kvs_txn.c	60.5% <0%> (-0.64%)	⬇️
src/broker/overlay.c	71.67% <0%> (-0.35%)	⬇️
... and 13 more

[coveralls]

Coverage decreased (-0.03%) to 77.92% when pulling 252d105 on chu11:kvs_oom_corner_case_fix into 38fc4a6 on flux-framework:master.

[garlick]

Ah that makes sense. Thanks.