Changes from commit 79db3aced2ffca84a696192343d5b811833eb671 were done in an attempt to avoid lock upgrade/downgrade problems for a conflicting F_SETLKW lock request as explained below.

Consider the following sequence of events acting upon a file:
    1. App1 requests shared blocking byte range lock request
       from offset 0 to 5 and is granted.
    2. App2 requests shared blokcing byte range lock request
       from offset 0 to 5 and is granted.
    3. App1 requests exclusive blocking byte range lock request
       from offset 0 to 5 and is blocked.

Before the specified commit hash eventhough App2 releases its lock, App1's blocked lock request could not be granted due to lack of lock owner and related checks inside first_overlap() which is invoked from grant_blocked_locks() as soon as the release happens from App2. This undefined wait/hang was fixed via the commit in such a way that we unlock App1's previously held lock as soon as we receive a blocking lock request from App1 which conflicts with App2's lock. This would mean that we break the promise of granted shared lock given to App1 temporarily. pl_send_prelock_unlock() implements this early unlock.