[minfs] Pre-reserve inodes/blocks to mitigate mid-op failures.

The most common failure for a Minfs operation (in particular for
writes) occurs when we attempt but are unable to allocate a new
block or inode. To ensure that this kind of error cannot happen
partway through an operation and leave the in-memory metadata in
an inconsistent state, the maximum number of blocks required for
an operation will be "reserved" prior to any state-changing calls.

In many cases the reserved elements will be an over-estimated, in
which case they will be unreserved at the end of the operation.

Additionally, in cases where a directory entry must be appended in
conjunction with other updates, ensure that the directory has
space for the new entry before making any other changes.

Test: TestFullOperations in utest/fs/test-minfs

Change-Id: I16bad01344723dc6b1dddfd2a04c919846032597

[fzl] Move fs::MappedVmo to fzl::MappedVmo

This is a useful utility outside of the scope of just filesystems.

Change-Id: Id9cfa3d40e7d45dd6dd94242c1a85204f1139bfa
Tested: Run `runtests`

[minfs] Refactor allocator, inode manager, and superblock

The MinFS allocators should be opaque entities, only
leaking details of on-disk format during initialization.

This patch reduces the reliance on callbacks into the Minfs
superclass, instead providing ownership of the fields
which need to be updated to the underlying allocator.

Test: /boot/test/fs/fs-test -f minfs

Change-Id: I50090c17a2e2b7487aa072860af1286f3ab53e0b