gfs2: split holders_compatible off from may_grant

Split off a section of may_grant() into its own function called
holders_compatible(). This makes the code easier to read and understand.

Replace may_grant() with holders_compatible() in
demote_incompat_holders(): we know that current_gh cannot be NULL here.

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