MB-18747 Don't grab spinlock when decrementing a file's ref counter
MB-18747 Don't grab spinlock when decrementing a file's ref counter

As a file ref counter is an atomic variable, we don't need to grab
the spinlock upon decrementing it.

Change-Id: I07bd6010bb5df9f34ce9417e78f12273742c11d5
Reviewed-on: http://review.couchbase.org/61862
Reviewed-by: Jung-Sang Ahn <e5ecb2c8d8dfc8277aaaee7c50855d74a54a4afa@gmail.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

couchbase/forestdb,wurikiji/forestdb,couchbase/forestdb,wurikiji/forestdb,hisundar/forestdb,hisundar/forestdb