common: fix few tests to work under bullseye

Fix execute tests under coverage tool bullseye:
-vmem_valgrind
-blk_nblock
-blk_rw
blk: check return value of pthread_mutex_unlock()
blk: add checking number and size of internal lbas in each arena
benchmarks: fix pread and pwrite

On some systems the pread and pwrite calls are not declared
due to lack of appropriate feature test macro.
Define __USE_UNIX98 before including unistd.h in order to turn on
pread and pwrite declarations.
Merge pull request #46 from plebioda/fix-pread-pwrite

benchmarks: fix pread and pwrite
Merge pull request #45 from ldorau/blk-check-return-value

blk: check return value of pthread_mutex_unlock()
Merge pull request #44 from tomaszkapela/pmemblk-fix-excessive-block-size

blk: add checking number and size of internal lbas in each arena
Merge pull request #37 from spawlows/fix-test-under-bullseye

common: fix few tests to work under bullseye