ext4: remove redundant GET_BLOCKS_CONVERT code

Remove the redundant code assigning values to ext4_map_blocks components
in ext4_ext_handle_unwritten_extents() for the EXT4_GET_BLOCKS_CONVERT
case, using the code at the function exit instead.  Clean up and reorder
that code to eliminate more redundancy and improve readability.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20200430185320.23001-3-enwlinux@gmail.com


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