wayland: unlink shm file earlier in create function

wayland: unlink shm file earlier in create function

create_shm_pool unlinks the temporary file a little,
too late. It should be unlinked before ftruncate()
is called for two reasons:

1) if ftruncate fails, the file is currently not
getting cleaned up at all
2) in theory, if the file is public some other process
could muck with it

This commit just moves the unlink call a little higher
up.

https://bugzilla.gnome.org/show_bug.cgi?id=760897