LU-9341 lod: Add special O_APPEND striping

LU-9341 lod: Add special O_APPEND striping

Files opened with O_APPEND are almost always log files,
which generally stay small and do not benefit from being
striped widely.  Additionally, PFL files accessed with
O_APPEND are fully instantiated, meaning that because the
files usually stay small, these objects usually wasted.

This patch adds special striping for files created with
O_APPEND.  This is controlled on the MDS by two new proc
variables:
mdd_append_stripe_count
mdd_append_pool

If the stripe count is set to 0 and the pool is not set,
this functionality is disabled and files created with
O_APPEND will be striped like any other file.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I433d1b8c80488a851b8eb26c78cf5519a6cd75bf
Reviewed-on: https://review.whamcloud.com/35617
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>