bvec: avoid variable shadowing warning

Due to the (indirect) nesting of min(..., min(...)), sparse will
show a variable shadowing warning whenever bvec.h is included.

Avoid that by assigning the inner min() to a temporary variable first.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>