gfs2: Initialize gh_error in gfs2_glock_nq

The gh_error field if a glock holder is initialized to zero in
gfs2_holder_init().  When a locking operation fails, gh_error is set to
an error code; when it succeeds, the gh_error value is left unchanged.
The field isn't initialized in gfs2_holder_reinit(), which is a problem.
Instead of fixing that directly, initialize gh_error in gfs2_glock_nq().
That also obsoletes the assignment in do_flock().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>