Remove dependency on linux/ext2_fs.h

Remove dependency on linux/ext2_fs.h

Due to a missing header in (some versions of) linux/ext2_fs.h,
configure's AC_CHECK_HEADERS test fails for this header, making it
appear "not found", and causing Linux-attr support to be disabled in bup
for no good reason.

Given that all the flags bup needs are available in linux/fs.h, we can
safely remove the requirement for linux/ext2_fs.h altogether, to work
around the problem.

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>