mm: vmalloc: update outdated comment for renamed vread()

mm: vmalloc: update outdated comment for renamed vread()

The function vread() was renamed to vread_iter() in commit 4c91c07c93bb
("mm: vmalloc: convert vread() to vread_iter()"), converting from a
buffer-based to an iterator-based interface.

Update the kdoc of vread_iter() to reflect the new interface: replace
references to @buf with @iter, drop the stale "kernel's buffer"
requirement, and update the self-reference from vread() to vread_iter().

Also update the stale vread() reference in pstore's ram_core.c.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Link: https://lkml.kernel.org/r/20260321105820.7134-1-kexinsun@smail.nju.edu.cn
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc: Julia Lawall <julia.lawall@inria.fr>
Cc: Kees Cook <kees@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>