hugetlb: move vm_fault declaration to the top of hugetlb_fault()

hugetlb: move vm_fault declaration to the top of hugetlb_fault()

hugetlb_fault() currently defines a vm_fault to pass to the generic
handle_userfault() function.  We can move this definition to the top of
hugetlb_fault() so that it can be used throughout the rest of the hugetlb
fault path.

This will help cleanup a number of excess variables and function arguments
throughout the stack.  Also, since vm_fault already has space to store the
page offset, use that instead and get rid of idx.

Link: https://lkml.kernel.org/r/20240221234732.187629-3-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>