mm,tmpfs: consider end of file write in shmem_is_huge

Take the end of a file write into consideration when deciding whether or
not to use huge pages for tmpfs files when the tmpfs filesystem is mounted
with huge=within_size

This allows large writes that append to the end of a file to automatically
use large pages.

Doing 4MB sequential writes without fallocate to a 16GB tmpfs file with
fio.  The numbers without THP or with huge=always stay the same, but the
performance with huge=within_size now matches that of huge=always.

huge		before		after
4kB pages	1560 MB/s	1560 MB/s
within_size	1560 MB/s	4720 MB/s
always:		4720 MB/s	4720 MB/s

[[email protected]: coding-style cleanups]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Rik van Riel <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Tested-by: Baolin Wang <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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