Remove the last of behaviour-specific checks on EXT2_OS_LINUX

Remove the last of behaviour-specific checks on EXT2_OS_LINUX

If there is a feature check, we can just depend on the feature check.
If it is something that can't be checked via a feature flag, then
instead of checking for EXT2_OS_LINUX, we should instead check for
*NOT* EXT2_OS_HURD. since HURD is the special case.

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