IB/fmr_pool: Flush all dirty FMRs from ib_fmr_pool_flush()

Commit a3cd7d90 ("IB/fmr_pool: ib_fmr_pool_flush() should flush all
dirty FMRs") caused a regression for iSER and was reverted in
e5507736.

This change attempts to redo the original patch so that all used FMR
entries are flushed when ib_flush_fmr_pool() is called without
affecting the normal FMR pool cleaning thread.  Simply move used
entries from the clean list onto the dirty list in ib_flush_fmr_pool()
before letting the cleanup thread do its job.

Signed-off-by: Pete Wyckoff <pw@osc.edu>
Signed-off-by: Roland Dreier <rolandd@cisco.com>