zram: ZRAM_DEF_COMP should depend on ZRAM

When Compressed RAM block device support is disabled, the
CONFIG_ZRAM_DEF_COMP symbol still ends up in the generated config file:

CONFIG_ZRAM_DEF_COMP="unset-value"

While this causes no real harm, avoid polluting the config file by
adding a dependency on ZRAM.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 917a59e81c34 ("zram: introduce custom comp backends API")
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/block/zram/Kconfig

···
 
 config ZRAM_DEF_COMP
 	string
+	depends on ZRAM
 	default "lzo-rle" if ZRAM_DEF_COMP_LZORLE
 	default "lzo" if ZRAM_DEF_COMP_LZO
 	default "lz4" if ZRAM_DEF_COMP_LZ4