Fix a bug in r265255: only return NULL if the requested map wasn't found.

Submitted by:	Luke Chang-Hsien Tsai <luke.tw@gmail.com>
MFC after:	1 week
Fix bug that, assuming a/ is a root of NFS filesystem mounted on autofs,
prevented "mv a/from a/to" from working, while "cd a && mv from to" was ok.

PR:		192948
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Restore the correct value when disabling probes. Otherwise the instrumented
tracepoints would continue to generate traps, which would be ignored but
could consume noticeable amounts of CPU if, say, all functions in the kernel
were instrumented.

X-MFC-With:	r270067