os: drop unnecessary memcpy in wl_os_mremap_maymove

FreeBSD doesn't support mremap [1], so we have a fallback
implementation based on munmap+mmap. We memcpy from the old memory
region to the new one, however this is unnecessary because the new
mapping references the same file as the old one.

Use msync to make sure any pending write is flushed to the underlying
file before we map the new region.

[1]: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=59912



Signed-off-by: Simon Ser <contact@emersion.fr>