x86/platform/uv: Fix UBSAN array-index-out-of-bounds

When UBSAN is enabled, multiple array-index-out-of-bounds messages are
printed:

[ 0.000000] [ T0] UBSAN: array-index-out-of-bounds in arch/x86/kernel/apic/x2apic_uv_x.c:276:23
[ 0.000000] [ T0] index 1 is out of range for type '<unknown> [1]'
...
[ 0.000000] [ T0] UBSAN: array-index-out-of-bounds in arch/x86/kernel/apic/x2apic_uv_x.c:277:32
[ 0.000000] [ T0] index 1 is out of range for type '<unknown> [1]'
...
[ 0.000000] [ T0] UBSAN: array-index-out-of-bounds in arch/x86/kernel/apic/x2apic_uv_x.c:282:16
[ 0.000000] [ T0] index 1 is out of range for type '<unknown> [1]'
...
[ 0.515850] [ T1] UBSAN: array-index-out-of-bounds in arch/x86/kernel/apic/x2apic_uv_x.c:1344:23
[ 0.519851] [ T1] index 1 is out of range for type '<unknown> [1]'
...
[ 0.603850] [ T1] UBSAN: array-index-out-of-bounds in arch/x86/kernel/apic/x2apic_uv_x.c:1345:32
[ 0.607850] [ T1] index 1 is out of range for type '<unknown> [1]'
...
[ 0.691850] [ T1] UBSAN: array-index-out-of-bounds in arch/x86/kernel/apic/x2apic_uv_x.c:1353:20
[ 0.695850] [ T1] index 1 is out of range for type '<unknown> [1]'

One-element arrays have been deprecated:

https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays

Switch entry in struct uv_systab to a flexible array member to fix UBSAN
array-index-out-of-bounds messages.

sizeof(struct uv_systab) is passed to early_memremap() and ioremap(). The
flexible array member is not accessed until the UV system table size is used to
remap the entire UV system table, so changes to sizeof(struct uv_systab) have no
impact.

Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/aTxksN-3otY41WvQ@hpe.com