erofs: apply proper VMA alignment for memory mapped files on THP

erofs: apply proper VMA alignment for memory mapped files on THP

There are mainly two reasons that thp_get_unmapped_area() should be
used for EROFS as other filesystems:

 - It's needed to enable PMD mappings as a FSDAX filesystem, see
   commit 74d2fad1334d ("thp, dax: add thp_get_unmapped_area for pmd
   mappings");

 - It's useful together with large folios and
   CONFIG_READ_ONLY_THP_FOR_FS which enable THPs for mmapped files
   (e.g. shared libraries) even without FSDAX.  See commit 1854bc6e2420
   ("mm/readahead: Align file mappings for non-DAX").

Fixes: 06252e9ce05b ("erofs: dax support for non-tailpacking regular file")
Fixes: ce529cc25b18 ("erofs: enable large folios for iomap mode")
Fixes: e6687b89225e ("erofs: enable large folios for fscache mode")
Reviewed-by: Jingbo Xu <[email protected]>
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]