efi/capsule-loader: Replace kmap() with kmap_local_page()

kmap() is deprecated in favour of kmap_local_page(), as described in
Documentation/mm/highmem.rst.

The conversion is safe here. efi_capsule_write() maps the page and
releases it within the same call, on both the success and the fail_unmap
error path, so the mapping never escapes the thread that created it and
the stack-based unmap ordering is preserved. No atomic context is
involved: the page is allocated with alloc_page(GFP_KERNEL) just above,
and the copy_from_user() performed while the page is mapped is fine
because faults are permitted in a local kmap region.
efi_capsule_setup_info() also runs while the mapping is live, but only
reads through the pointer.

kunmap_local() is handed a pointer that has been advanced into the page,
which is fine as it masks the address back down to the page boundary.

Build-tested only, on i386 with CONFIG_HIGHMEM=y -- where
kmap_local_page() actually establishes a mapping rather than returning
the direct-map address -- and on x86_64. No new gcc or sparse warnings.

Assisted-by: LLM sparse
Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

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