arm64: mm: fix the count comments in compute_indices

'count - 1' is confusing and not comply with the real code running.
'count' actually represents the extra entries required, no need minus 1.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210518101405.1048860-3-aisheng.dong@nxp.com
Signed-off-by: Will Deacon <will@kernel.org>