xfs: compute buffer address correctly in xmbuf_map_backing_mem

Prior to commit e614a00117bc2d, xmbuf_map_backing_mem relied on
folio_file_page to return the base page for the xmbuf's loff_t in the
xfile, and set b_addr to the page_address of that base page.

Now that folio_file_page has been removed from xmbuf_map_backing_mem, we
always set b_addr to the folio_address of the folio. This is correct
for the situation where the folio size matches the buffer size, but it's
totally wrong if tmpfs uses large folios. We need to use
offset_in_folio here.

Found via xfs/801, which demonstrated evidence of corruption of an
in-memory rmap btree block right after initializing an adjacent block.

Fixes: e614a00117bc2d ("xfs: cleanup mapping tmpfs folios into the buffer cache")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>