e2fsprogs: disable e4defrag on avr32

The fallocate syscall is not available on avr32. This is needed by the e4defrag
utility, so we disable this on avr32.

Fixes build failures such as the following.

  http://autobuild.buildroot.net/results/dcb/dcb4e5c6981a9299a2eb18e325d4de621846afdb/



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>