f2fs: use dget_parent and file_dentry in f2fs_file_open

f2fs: use dget_parent and file_dentry in f2fs_file_open

This patch synced with the below two ext4 crypto fixes together.

In 4.6-rc1, f2fs newly introduced accessing f_path.dentry which crashes
overlayfs. To fix, now we need to use file_dentry() to access that field.

Fixes: 5bf3a0769f55 ("ext4: use file_dentry()")
Fixes: 15ed41a15745 ("ext4: use dget_parent() in ext4_file_open()")
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>