fusefs: quiet some cache-related warnings

If the FUSE server does something that would make our cache incoherent,
we should print a warning to the user. However, we previously warned in
some situations when we shouldn't, such as if the file's size changed on
the server _after_ our own attribute cache had expired. This change
suppresses the warning in cases like that. It also moves the warning
logic to a single place within the code.

PR: 256936
Reported by: Agata <chogata@moosefs.pro>
Tested by: Agata <chogata@moosefs.pro>, jSML4ThWwBID69YC@protonmail.com
MFC after: 2 weeks