fusefs: don't panic if FUSE_GETATTR fails durint VOP_GETPAGES

During VOP_GETPAGES, fusefs needs to determine the file's length, which
could require a FUSE_GETATTR operation. If that fails, it's better to
SIGBUS than panic.

MFC after: 1 week
Sponsored by: Axcient
Reviewed by: markj, kib
Differential Revision: https://reviews.freebsd.org/D31994