btrfs: move space_info_flag_to_str() to space-info.h

btrfs: move space_info_flag_to_str() to space-info.h

Move space_info_flag_to_str() to space-info.h and as it now isn't static
to space-info.c any more prefix it with 'btrfs_'.

This way it can be re-used in other places.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>