block: fix get_max_segment_size() warning

block: fix get_max_segment_size() warning

Correct the parameter name in the comment of get_max_segment_size()
to fix following warning:-

block/blk-merge.c:220: warning: Function parameter or struct member 'len' not described in 'get_max_segment_size'
block/blk-merge.c:220: warning: Excess function parameter 'max_len' description in 'get_max_segment_size'

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240709045432.8688-1-kch@nvidia.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>