fseg_page_is_free(): Because MDEV-24167 changed fil_space_t::latch
to a simple non-recursive rw-lock, we must avoid acquiring a shared
latch if the current thread already holds an exclusive latch.
This affects the test innodb.innodb_bug59733, which is exercising
the change buffer.

fil_space_t::is_owner(): Make available in non-debug builds.