watch_queue: Fix NULL dereference in error cleanup

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

watch_queue: Fix NULL dereference in error cleanup

In watch_queue_set_size(), the error cleanup code doesn't take account of

the fact that __free_page() can't handle a NULL pointer when trying to free

up buffer pages that did get allocated.

Fix this by only calling __free_page() on the pages actually allocated.

Without the fix, this can lead to something like the following:

BUG: KASAN: null-ptr-deref in __free_pages+0x1f/0x1b0 mm/page_alloc.c:5473

Read of size 4 at addr 0000000000000034 by task syz-executor168/3599

...

Call Trace:

<TASK>

__dump_stack lib/dump_stack.c:88 [inline]

dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106

__kasan_report mm/kasan/report.c:446 [inline]

kasan_report.cold+0x66/0xdf mm/kasan/report.c:459

check_region_inline mm/kasan/generic.c:183 [inline]

kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189

instrument_atomic_read include/linux/instrumented.h:71 [inline]

atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]

page_ref_count include/linux/page_ref.h:67 [inline]

put_page_testzero include/linux/mm.h:717 [inline]

__free_pages+0x1f/0x1b0 mm/page_alloc.c:5473

watch_queue_set_size+0x499/0x630 kernel/watch_queue.c:275

pipe_ioctl+0xac/0x2b0 fs/pipe.c:632

vfs_ioctl fs/ioctl.c:51 [inline]

__do_sys_ioctl fs/ioctl.c:874 [inline]

__se_sys_ioctl fs/ioctl.c:860 [inline]

__x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x44/0xae