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.

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

(cherry picked from commit 4f917847c9037d9b76de188c03e13b81224431b2)