This reverts a8877df135c0395285945b11a75d57e10b7aecdf.
Previously, the "unmergeable" flag was necessary for the RAMFS,
because if the last vnode reference was released while there
was still a consumer (as the old ordering of _RemoveConsumer
had), then the release of the cache reference when the vnode
was removed would result in the cache trying to merge with
its now-only consumer and sole referrer.
Now, instead, we remove the consumer before releasing the store
reference, so that there's no chance the cache will be merged
inside this method.
mmap_cut_tests still pass, web browsers using ramfs shared_memory
still seem to work.
f836917f47