Disallow fcntl(F_READAHEAD) when the vnode is not a regular file.

The mountpoint may not have defined an iosize parameter, so an attempt
to configure readahead on a device file can lead to a divide-by-zero
crash.

The sequential heuristic is not applied to I/O to or from device files,
and posix_fadvise(2) returns an error when v_type != VREG, so perform
the same check here.

Reported by: syzbot+e4b682208761aa5bc53a@syzkaller.appspotmail.com
Reviewed by: kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21864