ANDROID: f2fs: Fix the calculation of the number of zones

Commit b50e6a61 ("ANDROID: Revert "f2fs: ensure only power of 2 zone
sizes are allowed"") did not update all code that assumes that the zone
size is a power of two. Hence this (untested) patch.

This issue was discovered by running the following grep command:

    git grep -nH ' & .*zone.*- 1'

Bug: 269471019
Change-Id: I9aa4cd1e621f34ed774d89d4f9763981c4b95f50
Signed-off-by: Bart Van Assche <bvanassche@google.com>