Count write times when extending relation files for shared buffers

Count write times when extending relation files for shared buffers

Relation files extended multiple blocks at a time have been counting the
number of blocks written, but forgot that to increment the write time in
this case, as write and relation extension are treated as two different
I/O operations in the shared stats: IOOP_EXTEND vs IOOP_WRITE.  In this
case IOOP_EXTEND was forgotten for normal (non-temporary) relations.

Write times are tracked when track_io_timing is enabled, which is not
the case by default.

Author: Nazir Bilal Yavuz
Reviewed-by: Robert Haas, Melanie Plageman
Discussion: https://postgr.es/m/CAN55FZ19Ss279mZuqGbuUNxka0iPbLgYuOQXqAKewrjNrp27VA@mail.gmail.com
Backpatch-through: 16