gfs2: Fix initialisation of args for remount

When gfs2 was converted to use fs_context, the initialisation of the
mount args structure to the currently active args was lost with the
removal of gfs2_remount_fs(), so the checks of the new args on remount
became checks against the default values instead of the current ones.
This caused unexpected remount behaviour and test failures (xfstests
generic/294, generic/306 and generic/452).

Reinstate the args initialisation, this time in gfs2_init_fs_context()
and conditional upon fc->purpose, as that's the only time we get control
before the mount args are parsed in the remount process.

Fixes: 1f52aa08 ("gfs2: Convert gfs2 to fs_context")
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>