Revert "nfs: don't reuse partially completed requests in nfs_lock_and_join_requests"

This reverts commit b571cfcb.

This patch appears to assume that if one request is complete, then the
others will complete too before unlocking. That is not a valid
assumption, since other requests could hit a non-fatal error or a short
write that would cause them not to complete.

Reported-by: Igor Raits <igor@gooddata.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=219508


Fixes: b571cfcb ("nfs: don't reuse partially completed requests in nfs_lock_and_join_requests")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>