f2fs crypto: avoid unneeded memory allocation in ->readdir

f2fs crypto: avoid unneeded memory allocation in ->readdir

commit e06f86e61d7a67fe6e826010f57aa39c674f4b1b upstream.

When decrypting dirents in ->readdir, fscrypt_fname_disk_to_usr won't
change content of original encrypted dirent, we don't need to allocate
additional buffer for storing mirror of it, so get rid of it.

[This backport fixes a regression in 4.4-stable caused by commit
11a6e8f89521 ("f2fs: check memory boundary by insane namelen"), which
depended on this missing commit.  This bad backport broke f2fs
encryption because it moved the incrementing of 'bit_pos' to earlier in
f2fs_fill_dentries() without accounting for it being used in the
encrypted dir case.  This caused readdir() on encrypted directories to
start failing.  Tested with 'kvm-xfstests -c f2fs -g encrypt'.]

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>