btrfs: prefix sysfs attribute struct names

Currently struct names for sysfs are generated only based on the
attribute names. This means that attribute names cannot be reused in
multiple places throughout the complete btrfs sysfs hierarchy.

E.g. allocation/data/total_bytes and allocation/data/single/total_bytes
result in the same struct name btrfs_attr_total_bytes. A workaround for
this case was made in the past by ad hoc creating an extra macro
wrapper, BTRFS_RAID_ATTR, that inserts some extra text in the struct
name.

Instead of polluting sysfs.h with such kind of extra macro definitions,
and only doing so when there are collisions, use a prefix which gets
inserted in the struct name, so we keep everything nicely grouped
together by default.

Current collections of attributes are:
* (the toplevel, empty prefix)
* allocation
* space_info
* raid
* features

Signed-off-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>