Commit a9805106 (table: seal_active_memtable: handle ENOSPC error)
made memtable flushing code stand ENOSPC and continue flusing again
in the hope that the node administrator would provide some free space.
However, it looks like the IO code may report back ENOSPC with some
exception type this code doesn't expect. This patch tries to fix it
refs: #11245
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>