mm/hugetlb: move hugetlb_sysctl_init() to the __init section

mm/hugetlb: move hugetlb_sysctl_init() to the __init section

hugetlb_sysctl_init() is only invoked once by an __init function and is
merely a wrapper around another __init function so there is not reason to
keep it.

Fixes the following warning when toning down some GCC inline options:

 WARNING: modpost: vmlinux: section mismatch in reference:
   hugetlb_sysctl_init+0x1b (section: .text) ->
     __register_sysctl_init (section: .init.text)

Link: https://lkml.kernel.org/r/20250319060041.2737320-1-marc.herbert@linux.intel.com
Signed-off-by: Marc Herbert <Marc.Herbert@linux.intel.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>