ext4: Merge ext4_da_get_block_write() into mpage_da_map_blocks()

The static function ext4_da_get_block_write() was only used by
mpage_da_map_blocks().  So to simplify the code, merge that function
into mpage_da_map_blocks().

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>