ext2: use common file type conversion

Deduplicate the ext2 file type conversion implementation and remove
EXT2_FT_* definitions - file systems that use the same file types as
defined by POSIX do not need to define their own versions and can
use the common helper functions decared in fs_types.h and implemented
in fs_types.c

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Jan Kara <jack@suse.cz>