bpf: fix mark_reg_unknown_value for spilled regs on map value marking

Martin reported a verifier issue that hit the BUG_ON() for his
test case in the mark_reg_unknown_value() function:

  [  202.861380] kernel BUG at kernel/bpf/verifier.c:467!
  [...]
  [  203.291109] Call Trace:
  [  203.296501]  [<ffffffff811364d5>] mark_map_reg+0x45/0x50
  [  203.308225]  [<ffffffff81136558>] mark_map_regs+0x78/0x90
  [  203.320140]  [<ffffffff8113938d>] do_check+0x226d/0x2c90
  [  203.331865]  [<ffffffff8113a6ab>] bpf_check+0x48b/0x780
  [  203.343403]  [<ffffffff81134c8e>] bpf_prog_load+0x27e/0x440
  [  203.355705]  [<ffffffff8118a38f>] ? handle_mm_fault+0x11af/0x1230
  [  203.369158]  [<ffffffff812d8188>] ? security_capable+0x48/0x60
  [  203.382035]  [<ffffffff811351a4>] SyS_bpf+0x124/0x960
  [  203.393185]  [<ffffffff810515f6>] ? __do_page_fault+0x276/0x490
  [  203.406258]  [<ffffffff816db320>] entry_SYSCALL_64_fastpath+0x13/0x94

This issue got uncovered after the fix in a08dd0da ("bpf: fix
regression on ver...