f2fs: compress: fix to disallow temp extension

f2fs: compress: fix to disallow temp extension

This patch restricts to configure compress extension as format of:

 [filename + '.' + extension]

rather than:

 [filename + '.' + extension + (optional: '.' + temp extension)]

in order to avoid to enable compression incorrectly:

1. compress_extension=so
2. touch file.soa
3. touch file.so.tmp

Fixes: 4c8ff7095bef ("f2fs: support data compression")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>