bcachefs: fix incorrect show_options results

When call show_options in bcachefs, the options buffer is appeneded
to the seq variable. In fact, it requires an additional comma to be
appended first. This will affect the remount process when reading
existing mount options.

Fixes: 9305cf91d05e ("bcachefs: bch2_opts_to_text()")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>