If fifo_open() is called with a negative file descriptor, return EINVAL

If fifo_open() is called with a negative file descriptor, return EINVAL
rather than panicking later.  This can occur if the kernel calls
vn_open() on a fifo, as there will be no associated file descriptor,
and therefore the file descriptor operations cannot be modified to
point to the fifo operation set.

MFC after:	3 days
Reported by:	Martin <nakal at nurfuerspam dot de>
PR:		94278