ext4: use ext4_ext_next_allocated_block instead of mext_next_extent

This allows us to make mext_next_extent static and potentially get rid
of it.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/extents.c
Simple merge

diff --cc fs/ext4/move_extent.c
Simple merge