When compiling with gcc version 12, this change also avoids the
compiler warning "infinite recursion detected" for runcmd()

This reverts commit 759bb341.

The legacy interface is confusing. It's better to make virtio_disk
conform to the virtio spec. This is supported in QEMU since 4.2 by
disabling force-legacy for virtio-mmio.

Matúš Jókay <matus.jokay@stuba.sk>)

The terminating NULL string is already included (see sys_exec()).

If va % PGSIZE == 0 and size == 0, we have last < a at the start of
the loop so we might never terminate.  That being said, this
particular case couldn't be triggered for now.

The inode cache isn't really a cache. The main purpose of it is to
allow for synchronization (locking individual inodes), providing
long-lived references to inodes, and ensuring that there is only inode
in memory.