null_blk: Fix the description of the cache_size module argument

null_blk: Fix the description of the cache_size module argument

When executing modinfo null_blk, there is an error in the description
of module parameter mbps, and the output information of cache_size is
incomplete.The output of modinfo before and after applying this patch
is as follows:

Before:
[...]
parm:           cache_size:ulong
[...]
parm:           mbps:Cache size in MiB for memory-backed device.
		Default: 0 (none) (uint)
[...]

After:
[...]
parm:           cache_size:Cache size in MiB for memory-backed device.
		Default: 0 (none) (ulong)
[...]
parm:           mbps:Limit maximum bandwidth (in MiB/s).
		Default: 0 (no limit) (uint)
[...]

Fixes: 058efe000b31 ("null_blk: add module parameters for 4 options")
Signed-off-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>