Depend on the kernel headers, not the ext2 headers, for Linux attr support.

Depend on the kernel headers, not the ext2 headers, for Linux attr support.

Switch to the kernel FS_* defines (in linux/ext2_fs.h) instead of the
EXT* defines (in ext2fs/ext2_fs.h -- from the ext2fs headers).

Also add FS_NOCOW_FL to the "modifiable flags" mask whenever it's
available.

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