fuse: replace folio_test_highmem() with folio_test_partial_kmap()

fuse_copy_folio() maps the folio with kmap_local_folio() and limits the
copy to the rest of the mapped page when the folio is in highmem, since
kmap_local_folio() only maps one page in that case.

folio_test_highmem() is not the most accurate test. On a highmem build
with CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP, kmap_local_page() maps just one
page even for lowmem folios, where folio_test_highmem() returns false
(see __kmap_local_page_prot() in mm/highmem.c).

The other highmem.h helpers that walk more than one page of a folio
(memcpy_from_folio(), memcpy_to_folio(), folio_zero_tail(), etc) all use
folio_test_partial_kmap() instead of folio_test_highmem() as well. Fuse
should use it too.

This is a no-op today since with order-0 folios, offset + count never
exceeds PAGE_SIZE. It matters once a descriptor can span more than one
page, which can happen once fuse enables large folios. Without this
change, fuse_copy_do() would copy past the end of the mapping on a
highmem build with CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

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