Btrfs: make profile_is_valid() check more strict

"0" is a valid value for an on-disk chunk profile, but it is not a valid
extended profile.  (We have a separate bit for single chunks in extended
case)

Also rename it to alloc_profile_is_valid() for clarity.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>