Inappropriate kernel memory exposure

[jserv]

Use make check to reproduce.

Reported by dmesg:

Bad or missing usercopy whitelist? Kernel memory exposure attempt detected from SLUB object 'simplefs_cache' (offset 4, size 4)!
Call Trace:
 __check_heap_object+0xb7/0x120
 __check_object_size+0xdb/0x1b0
 readlink_copy+0x4c/0x80
 vfs_readlink+0x7e/0x110
 do_readlinkat+0xf8/0x120
 __x64_sys_readlink+0x1f/0x30
 do_syscall_64+0x5a/0x130
 entry_SYSCALL_64_after_hwframe+0x44/0xa9