mke2fs, tune2fs, resize2fs: Use floating point to calculate percentages

mke2fs, tune2fs, resize2fs: Use floating point to calculate percentages

When calculating the number reserved blocks, use floating point for
better accuracy, since for big filesystems it really makes a
difference.  In addition, mke2fs and tune2fs accepts a floating point
number from the user, so they should provide that level of accuracy.

Addresses-Debian-Bug: #452639

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