No one currently needs EXT4_INODE_CASEFOLD, but add it to keep the
EXT4_INODE_* definitions in sync with the EXT4_*_FL definitions.

Also make it clearer that the casefold flag is only for directories.
Signed-off-by: NEric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200510215252.87833-1-ebiggers@kernel.orgSigned-off-by: NTheodore Ts'o <tytso@mit.edu>