rcu: add variant of assign() for null pointers

There is no need for release memory ordering when assigning a null pointer
to an rcu pointer, since the null pointer cannot be dereferenced.  Add
a specialization of assign() that takes advantage of this fact.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build: allow FS size specification from command line

Introduced variable fs_size_mb - FS size in megabytes.
Useful for debugging on EC2.

Reviewed-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>