erofs: use read_mapping_page instead of sb_bread

erofs: use read_mapping_page instead of sb_bread

As Christoph said [1], "This seems to be your only direct
use of buffer heads, which while not deprecated are a bit
of an ugly step child.  So if you can easily avoid creating
a buffer_head dependency in a new filesystem I think you
should avoid it. "

[1] https://lore.kernel.org/r/20190902125109[email protected]/
Reported-by: Christoph Hellwig <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>