kernel: mips: fix init crash/bootloop on 64-bit systems

kernel: mips: fix init crash/bootloop on 64-bit systems

The malta subtargets for mips64 and mips64el fail to start the init process
at boot, resulting in a boot loop. The issue was raised and analyzed within
FS#3277. Investigation suggested code near the [vdso] memory area of the
process was long jumping into a region inaccessible to the process, e.g.

 init: - preinit -
 init: Launched preinit instance, pid=522
 do_page_fault(): sending SIGSEGV to init for invalid read access from 0000000000000360
 epc = 0000000000000360 in init[aaab42b000+4000]
 ra  = 000000fffee385e0 in
 Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
 Rebooting in 1 seconds..

Note the low-memory read access and epc are the same. Upstream kernel 5.6
included a relevant patch and discussion:

  * d3f703c4359f ("mips: vdso: fix 'jalr t9' crash in vdso code")

Disassembly of the failing kernel's vdso.so confirmed presence of the
telltale long jumps, e.g.:

00000000000007c0 <__vdso_clock_getres@@LINUX_2.6>:
 [...]
 7dc:   0320f809        jalr    t9
 [...]

Restore booting mips64/mips64el malta by backporting the above commit:
  * 310-v5.6-mips-vdso-fix-jalr-t9-crash-in-vdso-code.patch

Fixes: 54310a3aa02f ("malta: add kernel 5.4 config")
Fixes: FS#3277
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=3277

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>