memfd: Reduce some syscalls in the on-demand case (#3757)

* memfd: Reduce some syscalls in the on-demand case

This tweaks the internal organization of the `MemFdSlot` to avoid some
syscalls in the default case as well as opportunistically in the pooling
case. The two cases added here are:

* A `MemFdSlot` is now created with a specified initial size. For
  pooling this is 0 but for the on-demand case this can be non-zero.

* When `instantiate` is called with no prior image and the sizes match
  (as will be the case for on-demand allocation) then `mprotect` is
  skipped entirely.

* In the `clear_and_remain-ready` case the `mprotect` is skipped if the
  heap wasn't grown at all.

This should avoid ever using `mprotect` unnecessarily and makes the
ranges we `mprotect` a bit smaller as well.

* Review comments

* Tweak allow to apply to whole crate

crates/runtime/src/lib.rs

@@ -19,6 +19,7 @@
         clippy::use_self
     )
 )]
+#![cfg_attr(not(memfd), allow(unused_variables, unreachable_code))]
 
 use std::sync::atomic::AtomicU64;