gfs2: make gfs2_holder.gh_owner_pid be a struct pid *

gfs2: make gfs2_holder.gh_owner_pid be a struct pid *

The gl_owner_pid field is used to get the holder task by its pid and check
whether the current is a holder, so make it in a proper manner, i.e.  via the
struct pid * manipulations.

Signed-off-by: Pavel Emelyanov <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Acked-by: Steven Whitehouse <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>