mm: readahead: improve mmap_miss heuristic for concurrent faults

If two or more threads of an application faulting on the same folio, the
mmap_miss counter can be decreased multiple times.  It breaks the
mmap_miss heuristic and keeps the readahead enabled even under extreme
levels of memory pressure.

It happens often if file folios backing a multi-threaded application are
getting evicted and re-faulted.

Fix it by skipping decreasing mmap_miss if the folio is locked.

This change was evaluated on several hundred thousands hosts in Google's
production over a couple of weeks.  The number of containers being stuck
in a vicious reclaim cycle for a long time was reduced several fold
(~10-20x), as well as the overall fleet-wide cpu time spent in direct
memory reclaim was meaningfully reduced.  No regressions were observed.

Link: https://lkml.kernel.org/r/20250815183224.62007-1-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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