ext4: scope ret locally in ext4_try_to_trim_range()

As commit 6920b391

 ("ext4: add new helper interface
ext4_try_to_trim_range()") moves some code into the separate function
ext4_try_to_trim_range(), the use of the variable ret within that
function is more limited and can be adjusted as well.

Scope the use of the variable ret locally and drop dead assignments.

No functional change.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210820120853.23134-1-lukas.bulwahn@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>