filemap: add filemap_map_folio_range()

filemap_map_folio_range() maps partial/full folio.  Comparing to original
filemap_map_pages(), it updates refcount once per folio instead of per
page and gets minor performance improvement for large folio.

With a will-it-scale.page_fault3 like app (change file write fault testing
to read fault testing.  Trying to upstream it to will-it-scale at [1]),
got 2% performance gain on a 48C/96T Cascade Lake test box with 96
processes running against xfs.

[1]: https://github.com/antonblanchard/will-it-scale/pull/37

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yin Fengwei <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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