mkfs.f2fs: avoid dumplicate extensions
mkfs.f2fs: avoid dumplicate extensions

Before copying an user specified extension to extension_list, check if it
is already in the list. User specified extensions are delimitted by token
either ',' or ' '.

Change-Id: I107542643f79f96dde19c1d3a9d375a771392acf
Signed-off-by: Sheng Yong <ce77ec10379c6f281208b7b41f03fd701be51d81@huawei.com>
Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

TeamExodus/external_f2fs-tools,TeamExodus/external_f2fs-tools