gfs2: Add GL_SKIP holder flag to dump_holder

Somehow, the GL_SKIP flag was missed when dumping glock holders.
This patch adds it to function hflags2str. I added it at the end because
I wanted Holder and Skip flags together to read "Hs" rather than "sH"
to avoid confusion with "Shared" ("SH") holder state.

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