uclient: don't use racy check for uncommitted data.

uclient: don't use racy check for uncommitted data.

Previously we used a check for if there were CEPH_CAP_FILE_BUFFER refs,
but that was racy if we had other threads (they could hold caps for
sync writes or something). Instead, see if we have any in-flight
writes or uncommitted objects.

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>