f2fs crypto: activate encryption support for fs APIs

This patch activates the following APIs for encryption support.

The rules quoted by ext4 are:
 - An unencrypted directory may contain encrypted or unencrypted files
   or directories.
 - All files or directories in a directory must be protected using the
   same key as their containing directory.
 - Encrypted inode for regular file should not have inline_data.
 - Encrypted symlink and directory may have inline_data and inline_dentry.

This patch activates the following APIs.
1. f2fs_link              : validate context
2. f2fs_lookup            :      ''
3. f2fs_rename            :      ''
4. f2fs_create/f2fs_mkdir : inherit its dir's context
5. f2fs_direct_IO         : do buffered io for regular files
6. f2fs_open              : check encryption info
7. f2fs_file_mmap         :      ''
8. f2fs_setattr           :      ''
9. f2fs_file_write_iter   :      ''           (Called by sys_io_submit)
10. f2fs_fallocate        : do not support fcollapse
11. f2fs_evict_inode      : free_encryption_info

Signed-off-by: Michael Halcrow <mhalcrow@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/inode.c

@@ -359,6 +359,10 @@ no_delete:
 	if (is_inode_flag_set(F2FS_I(inode), FI_UPDATE_WRITE))
 		add_dirty_inode(sbi, inode->i_ino, UPDATE_INO);
 out_clear:
+#ifdef CONFIG_F2FS_FS_ENCRYPTION
+	if (F2FS_I(inode)->i_crypt_info)
+		f2fs_free_encryption_info(inode);
+#endif
 	clear_inode(inode);
 }