mm/huge_memory: convert split_huge_pages_in_file() to use a folio

mm/huge_memory: convert split_huge_pages_in_file() to use a folio

Patch series "Remove FGP_HEAD flag".

We have just two users left of the FGP_HEAD flag and both of them are
better off; sometimes startlingly so as a result of conversion to use
folios.

This patch (of 4):

Removes a number of calls to compound_head() and a call to
pagecache_get_page().

Link: https://lkml.kernel.org/r/20221019183332.2802139-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20221019183332.2802139-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>