gfs2: Silence gcc format-truncation warning

Enlarge sd_fsname to be big enough for the longest long lock table name
and an arbitrary journal number.  This silences two -Wformat-truncation
warnings with gcc 7.1.1.

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