tmem: drop direct usage of opt_tmem

tmem: drop direct usage of opt_tmem

Most callers of tmem_freeable_pages() checked to see if by checking
opt_tmem before calling tmem_freeable_pages() but not all of them did. This
seemed like an oversight and to avoid similar situations like that,
stick the check of tmem into tmem_freeable_pages(). Similarly other
places should not directly check opt_tmem but instead use the
tmem_enabled() helper function.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>