fusefs: quiet some cache-related warnings

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