netdevsim: Fix memory leak of nsim_dev->fa_cookie

In the Linux kernel, the following vulnerability has been resolved:

netdevsim: Fix memory leak of nsim_dev->fa_cookie

kmemleak reports this issue:

unreferenced object 0xffff8881bac872d0 (size 8):
comm "sh", pid 58603, jiffies 4481524462 (age 68.065s)
hex dump (first 8 bytes):
04 00 00 00 de ad be ef ........
backtrace:
[<00000000c80b8577>] __kmalloc+0x49/0x150
[<000000005292b8c6>] nsim_dev_trap_fa_cookie_write+0xc1/0x210 [netdevsim]
[<0000000093d78e77>] full_proxy_write+0xf3/0x180
[<000000005a662c16>] vfs_write+0x1c5/0xaf0
[<000000007aabf84a>] ksys_write+0xed/0x1c0
[<000000005f1d2e47>] do_syscall_64+0x3b/0x90
[<000000006001c6ec>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

The issue occurs in the following scenarios:

nsim_dev_trap_fa_cookie_write()
kmalloc() fa_cookie
nsim_dev->fa_cookie = fa_cookie
..
nsim_drv_remove()

The fa_cookie allocked in nsim_dev_trap_fa_cookie_write() is not freed. To
fix, add kfree(nsim_dev->fa_cookie) to nsim_drv_remove().