[$] Freezing filesystems for suspend

Sometimes worms have a tendency to multiply once their can is opened.
James Bottomley recently encountered that situation; he led a session in
the filesystem track at the 2025 Linux Storage, Filesystem, Memory
Management, and BPF Summit (LSFMM+BPF) to discuss filesystem behavior with
respect to suspending and resuming the system. As he noted in his topic
proposal, he came at the problem because he needed a way to
resynchronize the contents of efivarfs
after a system resume and thought there should be an API available to use.
But, as the resulting thread shows, the filesystem freeze and thaw code had
never been used by the system-wide suspend and resume code. Due to a
scheduling mixup, though, several of us missed Bottomley's session,
including Luis Chamberlain who has been working on hooking those two pieces
up; what follows is largely from a second session that Chamberlain led,
with some background information from the topic-proposal discussion and an
email exchange with Bottomley.