staging: erofs: use common file type conversion

staging: erofs: use common file type conversion

Deduplicate the EROFS file type conversion implementation and
remove EROFS_FT_* definitions since it's the same as defined
by POSIX, let's follow ext2 as Linus pointed out [1]
commit e10892189428 ("ext2: use common file type conversion").

[1] https://lore.kernel.org/r/CAHk-=wiUs+b=iVKM3mVooXgVk7cmmC67KTmnAuL0cd_cMMVAKw@mail.gmail.com/

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190816071142.8633-1-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>