commitlog: avoid a syscall in a most common case of segment recycle
commitlog: avoid a syscall in a most common case of segment recycle

When recycling a segment in O_DSYNC mode if the size of the segment
is neither shrunk nor grown, avoid calling file::truncate() or
file::allocate().

Message-Id: <e5718b87a59ae68148ef4700e93d010bf16078f0@scylladb.com>

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