tmem: drop unnecessary lock in tmem_relinquish_pages()

tmem: drop unnecessary lock in tmem_relinquish_pages()

CID 1150562

tmem_rwlock is unnecessary in tmem_relinquish_pages(), as
such lock is used as gate for hypercalls. However
tmem_relinquish_pages deals with pages that are no longer
owned by any domain - hence there is no need for tmem_rwlock.

Also the function is protected by the 'heap_lock' which
is the only calleer of this function.

This patch drops said lock.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>