tcp: cleanup of syncache_expand()

• Consistently free the string after unlocking the sch, if possible.
• Remove the failure handling in case of sc != NULL, since this is not possible anymore.
• Remove the use of goto and instead return 0 in the three cases.

The only change in behavior is that in three out of the four cases,
where 0 is returned, *lsop is not set to NULL anymore. So the behavior
is now consistent and also documented in a comment. The current in
tree callers only look at *lsop, if and only if syncache_expand()
returns 1.

Reviewed by: Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52948