os/FileStore: return -ENODATA when removing non-existent xattr

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
osd/ReplicatedPG: fix OSD_OP_OMAPCLEAR

make OSD_OP_OMAPCLEAR only remove keys start with user prefix, not
affect any other key in DBObjectMap.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>