btrfs: reuse exit helper for cleanup in btrfs_bioset_init()

Do not duplicate the cleanup after failed initialization
in btrfs_bioset_init() and reuse the exit function btrfs_bioset_exit().

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>