ext2fs, blkid: localize environment-specific variables

ext2fs, blkid: localize environment-specific variables

Restructure the ext2fs_get_device_size() and blkid_get_dev_size()
code to localize the variables used for different device probing
methods.  This at least reduces the #ifdef mess to only one part
of the code for each method, and avoids "unused variable" compiler
warnings added when variables are declared without being #ifdef'd.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>