mds: fix owner check of file lock

flock and posix lock do not use process ID as owner identifier.
The process ID of who holds the lock is just for F_GETLK fcntl(2).
For linux kernel, File lock's owner identifier is the file pointer
through which the lock is requested.

The fix is do not take the 'pid_namespace' into consideration when
checking conflict locks. Also rename the 'pid' fields of struct
ceph_mds_request_args and struct ceph_filelock to 'owner', rename
'pid_namespace' fields to 'pid'.

The kclient counterpart of this patch modifies the flock code to
assign file pointer to the 'owner' field of lock message.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>