mm: shmem: rename mTHP shmem counters

The legacy PMD-sized THP counters at /proc/vmstat include thp_file_alloc,
thp_file_fallback and thp_file_fallback_charge, which rather confusingly
refer to shmem THP and do not include any other types of file pages.  This
is inconsistent since in most other places in the kernel, THP counters are
explicitly separated for anon, shmem and file flavours.  However, we are
stuck with it since it constitutes a user ABI.

Recently, commit 66f44583f9b6 ("mm: shmem: add mTHP counters for anonymous
shmem") added equivalent mTHP stats for shmem, keeping the same "file_"
prefix in the names.  But in future, we may want to add extra stats to
cover actual file pages, at which point, it would all become very
confusing.

So let's take the opportunity to rename these new counters "shmem_" before
the change makes it upstream and the ABI becomes immutable.  While we are
at it, let's improve the documentation for the legacy counters to make it
clear that they count shmem pages only.

Link: https://lkml.kernel.org/r/20240710095503.3193901-1-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lance Yang <ioworker0@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Daniel Gomez <da.gomez@samsung.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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