In munmap() downgrade the vm_map_lock to a read lock before taking a read
lock on the pmc-sx lock. This prevents a deadlock with
pmc_log_process_mappings, which has an exclusive lock on pmc-sx and tries
to get a read lock on a vm_map. Downgrading the vm_map_lock in munmap
allows pmc_log_process_mappings to continue, preventing the deadlock.

Without this change I could cause a deadlock on a multicore 8.1-RELEASE
system by having one thread constantly mmap'ing and then munmap'ing a
PROT_EXEC mapping in a loop while I repeatedly invoked and stopped pmcstat
in system-wide sampling mode.

Reviewed by: fabient
Approved by: emaste (mentor)
MFC after: 2 weeks