UPSTREAM: arm64: vdso: minor ABI fix for clock_getres

(cherry picked from commit e1b6b6ce55a0a25c8aa8af019095253b2133a41a)

The vdso implementation of clock_getres currently returns 0 (success)
whenever a null timespec is provided by the caller, regardless of the
clock id supplied.

This behavior is incorrect.  It should fall back to syscall when an
unrecognized clock id is passed, even when the timespec argument is
null.  This ensures that clock_getres always returns an error for
invalid clock ids.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Bug: 20045882
Bug: 63737556
Bug: 69626243
Change-Id: I93caac645d807403cab85245a33faf6439c6f3c1