os/FileStore: only adjust up op queue for btrfs

We only need to adjust up the op queue limits during commit for btrfs,
because the snapshot initiation (async create) is currently
high-latency and the op queue is quiesced during that period.

This lets us revert 44dca5c, which disabled the extra allowance because
it is generally bad for non-btrfs writeahead mode.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 38871e27eca5a34de78db23aa3663f6cb045d461)