Simplify some maths in xlogreader.c

Simplify some maths in xlogreader.c

An LSN was calculated from a segment number, a segment size and a
position offset, matching exactly the LSN given by the caller of
XLogReaderValidatePageHeader().  This change removes the extra LSN
calculation, relying only on the LSN given by the function caller
instead.

Author: Bharath Rupireddy
Reviewed-by: Richard Guo, Álvaro Herrera, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CALj2ACXuh4Ms9j9sxMYdtHEe=5sFcyrs-GAHyADu_A_G71kZTg@mail.gmail.com