vhost: fix kernel module insertion

Problem:if I firstly insert my kmod_test.ko, then insert eventfd_link.ko,
error will happen with hint "Device or resource busy". This is because
the default minor device number, 0, has been occupied by my kmod_test.ko .

root@distro:~/test$ lsmod
Module                  Size  Used by
kmod_test                927  0
vboxsf                 35930  4
vboxguest             222130  1 vboxsf
microcode              10315  0
autofs4                25051  0
root@distro:~/test$ insmod ./eventfd_link.ko
insmod: ERROR: could not insert module ./eventfd_link.ko: Device or
resource busy

Explanation: For miscdevices, the major device_no is same, so the minor
device_no should be set to ditinguish different misc devices;  if not set
the minor, it may fail while insmod due to the default minor value, 0, has
been used by other miscdevice. MISC_DYNAMIC_MINOR means to let Linux
kernel dynamically assign one minor devide number while loading.

Signed-off-by: Xiaobo Chi <xiaobo.chi@nokia.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>

lib/librte_vhost/eventfd_link/eventfd_link.c

@@ -249,6 +249,7 @@ static const struct file_operations eventfd_link_fops = {
 
 
 static struct miscdevice eventfd_link_misc = {
+	.minor = MISC_DYNAMIC_MINOR,
 	.name = "eventfd-link",
 	.fops = &eventfd_link_fops,
 };