fusefs: cache file attributes

FUSE_LOOKUP, FUSE_GETATTR, FUSE_SETATTR, FUSE_MKDIR, FUSE_LINK,
FUSE_SYMLINK, FUSE_MKNOD, and FUSE_CREATE all return file attributes with a
cache validity period. fusefs will now cache the attributes, if the server
returns a non-zero cache validity period.

This change does _not_ implement finite attr cache timeouts. That will
follow as part of PR 235773.

PR: 235775
Reported by: cem
Sponsored by: The FreeBSD Foundation