Use XLogFromFileName() in pg_resetwal to parse position from WAL file

Hi,

It looks like there's an opportunity to replace explicit WAL file
parsing code with XLogFromFileName() in pg_resetwal.c. This was not
done then (in PG 10) because the XLogFromFileName() wasn't accepting
file size as an input parameter (see [1]) and pg_resetwal needed to
use WAL file size from the controlfile. Thanks to the commit
fc49e24fa69a15efacd5b8958115ed9c43c48f9a which added the
wal_segsz_bytes parameter to XLogFromFileName().

I'm attaching a small patch herewith. This removes using extra
variables in pg_resetwal.c and a bit of duplicate code too (5 LOC).

Thoughts?

[1] https://github.com/postgres/postgres/blob/REL_10_STABLE/src/include/access/xlog_internal.h

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com