libext2fs: fix ext2fs_link() for EXT2FS_LINK_APPEND and non-regular files

libext2fs: fix ext2fs_link() for EXT2FS_LINK_APPEND and non-regular files

Fix the incorrect flag being passed to ext2fs_process_dir_block().
This bug was masked because EXT2_FT_REG_FILE has the same code point
as DIRENT_FLAG_INCLUDE_EMPTY which was the flag that was needed and
mke2fs -d was only use ext2fs_lik() for regular files.

Fixes: 53aa6c54224f ("libext2fs: add the EXT2FS_LINK_APPEND flag ...)
Signed-off-by: Theodore Ts'o <tytso@mit.edu>