uma: Avoid unmapping direct-mapped slabs

startup_alloc() uses pmap_map() to map slabs used for bootstrapping the
VM. pmap_map() may ignore the hint address and simply return a range
from the direct map. In this case we must not unmap the range in
startup_free().

UMA uses bootstart and bootmem to track the range of KVA into which
slabs are mapped if the direct map is not used. Unmap a startup slab
only if it was mapped into that range.

Reported by: alc
Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27885