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 74d2fad1 ("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 1854bc6e
   ("mm/readahead: Align file mappings for non-DAX").

Fixes: 06252e9c ("erofs: dax support for non-tailpacking regular file")
Fixes: ce529cc2 ("erofs: enable large folios for iomap mode")
Fixes: e6687b89 ("erofs: enable large folios for fscache mode")
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240306053138.2240206-1-hsiangkao@linux.alibaba.com

fs/erofs/data.c

@@ -447,5 +447,6 @@ const struct file_operations erofs_file_fops = {
 	.llseek		= generic_file_llseek,
 	.read_iter	= erofs_file_read_iter,
 	.mmap		= erofs_file_mmap,
+	.get_unmapped_area = thp_get_unmapped_area,
 	.splice_read	= filemap_splice_read,
 };