ext4: remove code duplication in ext4_get_block_write_nolock()

729f52c6 introduced function ext4_get_block_write_nolock() that
is very similar to _ext4_get_block(). Eliminate code duplication
by passing different flags to _ext4_get_block()

Tested: xfs tests

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>