sort: Cache value of MB_CUR_MAX

Every usage of MB_CUR_MAX results in a call to __mb_cur_max. This is
inefficient and redundant. Caching the value of MB_CUR_MAX in a global
variable removes these calls and speeds up the runtime of sort. For
numeric sorting, runtime is almost halved in some tests.

PR: 255551
PR: 255840
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30170