runtime/pprof: move common code to writeProfileInternal function

This patch provides changes according to TODO. Since writeMutex and
writeBlock functions have a lot of code in common, it is better to
move this code to one function.

Change-Id: I81aaad067b0cb1647824909f3b5f6861add3a7ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/280152


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>