client: rename flush_caps() with no arguments to flush_caps_sync()

client: rename flush_caps() with no arguments to flush_caps_sync()

Per Greg's recommendation, change the name of this function to better
indicate what it does now that we always request a journal flush on
the last cap flush.

Also, add a comment above the function to better explain why we do this.

Signed-off-by: Jeff Layton <jlayton@redhat.com>