gve: fix NULL dereference due to missing ptp adjfine

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

gve: fix NULL dereference due to missing ptp adjfine

Fix NULL dereference due to missing implementation of adjfine, which can be triggered from usermode as follows:

sudo ./testptp -d /dev/ptp0 -f 0 [ 551.943697] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] [ 552.061946] Call Trace: [ 552.064487] [ 552.066681] ptp_clock_adjtime+0x1c0/0x2c0 [ 552.070874] ? get_clock_desc+0x6b/0xb0 [ 552.074825] pc_clock_adjtime+0x78/0xc0 [ 552.078755] __do_sys_clock_adjtime+0x85/0x110 [ 552.083293] do_syscall_64+0xea/0x610