Remove LRU_STATE_RECLAIMING and always perform reclaim in lru_try_reap_entry()

Maintaining the LRU_STATE_RECLAIMING flag in the LRU thread makes it sensitive
to the scheduling of the LRU thread, which can be rare in case caching_fds
is off.
Instead, maintain entries_used atomic counter and check it again the hiwat
in lru_try_reap_entry()

Signed-off-by: Amit Dror <amitd@infinidat.com>
Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Set a lower limit for the LRU thread wait time

Signed-off-by: Amit Dror <amitd@infinidat.com>
Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Order and cleanup in LRU flags

Signed-off-by: Amit Dror <amitd@infinidat.com>
Signed-off-by: Matt Benjamin <matt@linuxbox.com>