libgeom: Avoid fixed remappings of the devstat device

libgeom maintains a quasi-private mapping of /dev/devstat, which might
grow over time if new devices appear. When the mapping needs to be
expanded, the old mapping is passed as a hint, but this appears to be
unnecessary.

Simplify and improve things a bit:

• stop passing a hint when remapping,
• don't creat a mapping in geom_stats_open(), as geom_stats_resync() will create it for us,
• check for errors from munmap().

Reviewed by: imp, asomers
Tested by: asomers
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D46294