btrfs: cleanup root usage by btrfs_get_alloc_profile

There are two places where we don't already know what kind of alloc
profile we need before calling btrfs_get_alloc_profile, but we need
access to a root everywhere we call it.

This patch adds helpers for btrfs_{data,metadata,system}_alloc_profile()
and relegates btrfs_system_alloc_profile to a static for use in those
two cases.  The next patch will eliminate one of those.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>