Btrfs: don't invoke btrfs_invalidate_inodes() in the spin lock context

btrfs_invalidate_inodes() may sleep, so we should not invoke it in the
spin lock context. Fix it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>