mm: make mmap_miss accounting symmetric for VM_SEQ_READ

mm: make mmap_miss accounting symmetric for VM_SEQ_READ

do_sync_mmap_readahead() skips both the mmap_miss increment and the
MMAP_LOTSAMISS check for VM_SEQ_READ mappings, since sequential access is
non-speculative and should always read ahead.  The two decrement sites in
do_async_mmap_readahead() and filemap_map_pages() do not mirror this skip,
so concurrent faults on a VM_SEQ_READ mapping can still drive
ra->mmap_miss down to zero through the decrement paths even though nothing
in the sync path ever increments it.  The counter itself is per-file
(file->f_ra.mmap_miss), so it can be moved by any VMA mapping the file,
not just the one currently faulting.

Skip the decrement for VM_SEQ_READ in both decrement sites so the counter
only moves for mappings that also participate in the increment side.  No
functional change for VM_SEQ_READ users, since the increment-side gate
already prevents the counter from being consulted on their behalf, but it
stops a VM_SEQ_READ mapping from biasing the counter for other mappings of
the same file.

Link: https://lore.kernel.org/20260525145751.2671248-1-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Closes: https://lore.kernel.org/all/8edc8cd0-f65c-4456-9b3f-362e744c9a96@linux.dev/
Reviewed-by: William Kucharski <william.kucharski@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>