jffs2: Use function instead of macro when initialize compressors

The initialized compressors should be released if one of them
initialize fail, this is the pre-patch for fix the problem, use
function instead of the macro in jffs2_compressors_init() to
simplify the codes, no functional change intended.

Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>