btrfs: add separate bounds checker for set/get helpers

The bounds checking is now done in map_private_extent_buffer but that
will be removed in following patches and some sanity checks should still
be done.

There are two separate checks to see the kind of out of bounds access:
partial (start offset is in the buffer) or complete (both start and end
are out).

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>