ext4: simplify calculation of blocks to free on error
ext4: simplify calculation of blocks to free on error

In ext4_ext_map_blocks(), if we have successfully allocated the data
blocks, but then run into trouble inserting the extent into the extent
tree, most likely due to an ENOSPC condition, determine the arguments
to ext4_free_blocks() in a simpler way which is easier to prove to be
correct.

Signed-off-by: "Theodore Ts'o" <4ed386e0495d3e109932df055831d9ec2f824927@mit.edu>


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