kernel: fix jffs2 compilation with GCC_PLUGIN_RANDSTRUCT enabled

kernel: fix jffs2 compilation with GCC_PLUGIN_RANDSTRUCT enabled

Designated initializers are required when using the randstruct GCC
plugin, otherwise an error like the following is seen:

./include/linux/lzma.h:60:31: error: positional initialization of field in 'struct' declared with 'designated_init' attribute [-Werror=designated-init]

This was originally applied via 55643e469c21, but was unintentionally
reverted in 483503603cb2.

Fixes: 483503603cb2 ("generic: 5.15: rework pending patch")
Signed-off-by: Matt Merhar <mattmerhar@protonmail.com>
(cherry picked from commit b2068f4aac43754a681b675ff3814d9ca87ac986)
[ drop change for unavailable kernel 6.1 ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>