gfs2: Fix unused value warning in do_gfs2_set_flags()

Coverity complains of an unused value:

CID 119623 (#1 of 1): Unused value (UNUSED_VALUE)
assigned_value: Assigning value -1 to error here, but that stored value is
overwritten before it can be used.
237        error = -EPERM;

Fix it by removing the assignment.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>