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

[markj]

Summary

f_iosize is 0 for devfs vnodes, resulting in a divide by zero if one
calls, for example, fcntl(STDIN_FILENO, F_READAHEAD, 4096).

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

[mjg]

sys/kern/kern_descrip.c

791

vnode is assigned the line prior

[markj]

Address mjg's note.