rts/linker: Fix out-of-bounds mapping logic

Previously the structure of `mmapInRegion` concealed a subtle bug
concerning handling of `mmap` returning mappings below the beginning of
the desired region. Specifically, we would reset `p = result + bytes`
and then again reset `p = region->start` before looping around for
another iteration. This resulted in an infinite loop on FreeBSD.

Fixes #25492.