xfs: use shmem_get_folio in xfile_obj_store

Switch to using shmem_get_folio and manually dirtying the page instead
of abusing aops->write_begin and aops->write_end in xfile_get_page.

This simplifies the code by not doing indirect calls of not actually
exported interfaces that don't really fit the use case very well, and
happens to get us large folio support for free.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>