filemap: optimize folio refount update in filemap_map_pages

There are two meaningless folio refcount update for order0 folio in
filemap_map_pages().  First, filemap_map_order0_folio() adds folio
refcount after the folio is mapped to pte.  And then, filemap_map_pages()
drops a refcount grabbed by next_uptodate_folio().  We could remain the
refcount unchanged in this case.

As Matthew metenioned in [1], it is safe to call folio_unlock() before
calling folio_put() here, because the folio is in page cache with refcount
held, and truncation will wait for the unlock.

Optimize filemap_map_folio_range() with the same method too.

With this patch, we can get 8% performance gain for lmbench testcase
'lat_pagefault -P 1 file' in order0 folio case, the size of file is 512M.


Link: https://lkml.kernel.org/r/20250904132737.1250368-1-tujinjiang@huawei.com
Link: https://lore.kernel.org/all/aKcU-fzxeW3xT5Wv@casper.infradead.org/ [1]
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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