WBThrottle: Call posix_fadvise to free page cache if nocache set in clear().

Because FileStore will sync fs and call clear() to clear all pending
ghobjct. So in clear(), the pending object can free the page cache if
nocache is set.
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
os: add filestore_fadvise to control whether use posix_fadvise.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
os/FileStore: merge filestore_replica_fadvise -> filestore_fadvise

Signed-off-by: Sage Weil <sage@redhat.com>
os/FileStore: stop disabling fadvise on XFS

This was an issue with mixing fadvise and sync_file_range, not with just
fadvise.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #3080 from ceph/wip-fadvise

osd: reenable fadvise; use it more often

sage-2014-12-16_06:27:10-rados:singleton-wip-sage-testing2-distro-basic-multi
sage-2014-12-15_13:18:23-rados:thrash-wip-sage-testing2-distro-basic-multi

Reviewed-by: Sage Weil <sage@redhat.com>