dirty_memory_manager: flush_permit: add has_sstable_write_permit
dirty_memory_manager: flush_permit: add has_sstable_write_permit

after release_sstable_write_permit is called,
_sstable_write_permit will have no value.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla