ext4: remove map_from_cluster from ext4_ext_map_blocks

We can use the variable allocated_clusters rather than map_from_clusters
to control reserved block/cluster accounting in ext4_ext_map_blocks.
This eliminates a variable and associated code and improves readability
a little.

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20200311205125.25061-1-enwlinux@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>