Make linux stat(2) return the same st_dev for every devfs instance.

Make linux stat(2) return the same st_dev for every devfs instance.
The reason for this is to work around an idiosyncrasy of glibc
getttynam(3) implementation: it checks whether st_dev returned for
fd 0 is the same as st_dev returned for the target of /proc/self/fd/0
symlink, and with linux chroots having their own devfs instance,
the check will fail if you chrooted into it.

PR:		kern/240767
Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25559