column_family: Re-acquire flush permit in case of error
column_family: Re-acquire flush permit in case of error

If we fail to flush an sstable, after creating the flush_reader, then
we will have released the flush permit when we retry the flush. Ensure
that when retrying, we re-acquire the flush permit.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla