Handle a NULL thread pointer in linux_close_file().

This can happen if a file is closed during unix socket GC. The same bug
was fixed for devfs descriptors in r228361.

PR: 242913
Reported and tested by: iz-rpi03@hs-karlsruhe.de
Reviewed by: hselasky, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23178