fusefs: fix a panic in vop_close with a CTL consumer

Kernel consumers like CTL may lack ucred objects. That led to a panic
when CTL closed a fuse file, if atime was enabled on the mountpoint.
Fix it by checking the ucred during close.

PR: 283402
MFC after: 2 weeks
Sponsored by: ConnectWise
Differential Revision: https://reviews.freebsd.org/D48165