zram: propagate error to user

zram: propagate error to user

When we initialized zcomp with single, we couldn't change
max_comp_streams without zram reset but current interface doesn't show
any error to user and even it changes max_comp_streams's value without
any effect so it would make user very confusing.

This patch prevents max_comp_streams's change when zcomp was initialized
as single zcomp and emit the error to user(ex, echo).

[[email protected]: don't return with the lock held, per Sergey]
[[email protected]: fix coccinelle warnings]
Signed-off-by: Minchan Kim <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Jerome Marchand <[email protected]>
Acked-by: Sergey Senozhatsky <[email protected]>
Signed-off-by: Fengguang Wu <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>