GFS2: Set s_mode before parsing mount options

In the generic mount_bdev() function, deactivate_locked_super() is
called after the fill_super() call fails, at which point s_mode has been
set. kill_block_super() expects this and dumps a warning when
FMODE_EXCL is not set in s_mode.

In gfs2_mount() we call deactivate_locked_super() on failure of
gfs2_mount_args(), at which point s_mode has not yet been set. This
causes kill_block_super() to dump a stack trace when gfs2 fails to mount
with invalid options. Set s_mode earlier in gfs2_mount() to avoid that.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>