[$] 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 inthe filesystem track at the 2025 Linux Storage, Filesystem, MemoryManagement, and BPF Summit (LSFMM+BPF) to discuss filesystem behavior withrespect to suspending and resuming the system. As he noted in his topicproposal, he came at the problem because he needed a way toresynchronize the contents of efivarfsafter 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 hadnever been used by the system-wide suspend and resume code. Due to ascheduling mixup, though, several of us missed Bottomley's session,including Luis Chamberlain who has been working on hooking those two piecesup; what follows is largely from a second session that Chamberlain led,with some background information from the topic-proposal discussion and anemail exchange with Bottomley.