client: only set MClientCaps::FLAG_SYNC when flushing dirty auth caps

client: only set MClientCaps::FLAG_SYNC when flushing dirty auth caps

Sidharth noticed that we were probably doing more FLAG_SYNC cap flushes
than necessary.

This flag is intended for sync/fsync type activity, and in those cases
all we care about is cleaning dirty caps. We have no need to expedite
other types of cap calls (releases, revokes, etc.).

Fixes: https://tracker.ceph.com/issues/44963
Reported-by: Sidharth Anupkrishnan <sanupkri@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>