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().