Filestore: If FileStoreBackend don't support or disable fiemap for
fiemap, it return -EOPNOTSUPP.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
ReplicatedPG: For getattrs firstly check the result.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
ReplicatedPG: Don't use the return value of getattr_maybe_cache as attr length.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>