KVM: arm64: Convert kvm_mpidr_index() to bitmap_gather()

Linux 6.9 has introduced new bitmap manipulation helpers, with
bitmap_gather() being of special interest, as it does exactly
what kvm_mpidr_index() is already doing.

Make the latter a wrapper around the former.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240502154247.3012042-1-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_host.h

@@ -220,20 +220,10 @@ struct kvm_mpidr_data {
 
 static inline u16 kvm_mpidr_index(struct kvm_mpidr_data *data, u64 mpidr)
 {
-	unsigned long mask = data->mpidr_mask;
-	u64 aff = mpidr & MPIDR_HWID_BITMASK;
-	int nbits, bit, bit_idx = 0;
-	u16 index = 0;
+	unsigned long index = 0, mask = data->mpidr_mask;
+	unsigned long aff = mpidr & MPIDR_HWID_BITMASK;
 
-	/*
-	 * If this looks like RISC-V's BEXT or x86's PEXT
-	 * instructions, it isn't by accident.
-	 */
-	nbits = fls(mask);
-	for_each_set_bit(bit, &mask, nbits) {
-		index |= (aff & BIT(bit)) >> (bit - bit_idx);
-		bit_idx++;
-	}
+	bitmap_gather(&index, &aff, &mask, fls(mask));
 
 	return index;
 }