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>
Revert "filestore: disable extra committing queue allowance"

This reverts commit 44dca5c8c5058acf9bc391303dc77893793ce0be.

The allowance is not only added for btrfs as of commit
e639254a0c5f8e3528fa8f2b2b451296653556bc, which makes us happy
for both non-btrfs (lower latency) and btrfs (better small io
throughput, no big stall during commit).

Signed-off-by: Sage Weil <sage@inktank.com>