f2fs: rework filename handling

Rework f2fs's handling of filenames to use a new 'struct f2fs_filename'.
Similar to 'struct ext4_filename', this stores the usr_fname, disk_name,
dirhash, crypto_buf, and casefolded name.  Some of these names can be
NULL in some cases.  'struct f2fs_filename' differs from
'struct fscrypt_name' mainly in that the casefolded name is included.

For user-initiated directory operations like lookup() and create(),
initialize the f2fs_filename by translating the corresponding
fscrypt_name, then computing the dirhash and casefolded name if needed.

This makes the dirhash and casefolded name be cached for each syscall,
so we don't have to recompute them repeatedly.  (Previously, f2fs
computed the dirhash once per directory level, and the casefolded name
once per directory block.)  This improves performance.

This rework also makes it much easier to correctly handle all
combinations of normal, encrypted, casefolded, and encrypted+casefolded
directories.  (The fourth isn't supported yet but is being worked on.)

The only other cases where an f2fs_filename gets initialized are for two
filesystem-internal operations: (1) when converting an inline directory
to a regular one, we grab the needed disk_name and hash from an existing
f2fs_dir_entry; and (2) when roll-forward recovering a new dentry, we
grab the needed disk_name from f2fs_inode::i_name and compute the hash.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>