Btrfs: fix lockdep error in async commit

Lockdep complains about btrfs's async commit:

[ 2372.462171] [ BUG: bad unlock balance detected! ]
[ 2372.462191] 3.12.0+ #32 Tainted: G        W
[ 2372.462209] -------------------------------------
[ 2372.462228] ceph-osd/14048 is trying to release lock (sb_internal) at:
[ 2372.462275] [<ffffffffa022cb10>] btrfs_commit_transaction_async+0x1b0/0x2a0 [btrfs]
[ 2372.462305] but there are no more locks to release!
[ 2372.462324]
[ 2372.462324] other info that might help us debug this:
[ 2372.462349] no locks held by ceph-osd/14048.
[ 2372.462367]
[ 2372.462367] stack backtrace:
[ 2372.462386] CPU: 2 PID: 14048 Comm: ceph-osd Tainted: G        W    3.12.0+ #32
[ 2372.462414] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS 080015  11/09/2011
[ 2372.462455]  ffffffffa022cb10 ffff88007490fd28 ffffffff816f094a ffff8800378aa320
[ 2372.462491]  ffff88007490fd50 ffffffff810adf4c ffff8800378aa320 ffff88009af97650
[ 2372.462526]  ffffffffa022cb10 ffff88007490fd88 ffffffff810b01ee ffff8800898c0000
[ 2372.462562] Call Trace:
[ 2372.462584]  [<ffffffffa022cb10>] ? btrfs_commit_transaction_async+0x1b0/0x2a0 [btrfs]
[ 2372.462619]  [<ffffffff816f094a>] dump_stack+0x45/0x56
[ 2372.462642]  [<ffffffff810adf4c>] print_unlock_imbalance_bug+0xec/0x100
[ 2372.462677]  [<ffffffffa022cb10>] ? btrfs_commit_transaction_async+0x1b0/0x2a0 [btrfs]
[ 2372.462710]  [<ffffffff810b01ee>] lock_release+0x18e/0x210
[ 2372.462742]  [<ffffffffa022cb36>] btrfs_commit_transaction_async+0x1d6/0x2a0 [btrfs]
[ 2372.462783]  [<ffffffffa025a7ce>] btrfs_ioctl_start_sync+0x3e/0xc0 [btrfs]
[ 2372.462822]  [<ffffffffa025f1d3>] btrfs_ioctl+0x4c3/0x1f70 [btrfs]
[ 2372.462849]  [<ffffffff812c0321>] ? avc_has_perm+0x121/0x1b0
[ 2372.462873]  [<ffffffff812c0224>] ? avc_has_perm+0x24/0x1b0
[ 2372.462897]  [<ffffffff8107ecc8>] ? sched_clock_cpu+0xa8/0x100
[ 2372.462922]  [<ffffffff8117b145>] do_vfs_ioctl+0x2e5/0x4e0
[ 2372.462946]  [<ffffffff812c19e6>] ? file_has_perm+0x86/0xa0
[ 2372.462969]  [<ffffffff8117b3c1>] SyS_ioctl+0x81/0xa0
[ 2372.462991]  [<ffffffff817045a4>] tracesys+0xdd/0xe2

====================================================

It's because that we don't do the right thing when checking if it's ok to
tell lockdep that we're trying to release the rwsem.

If the trans handle's type is TRANS_ATTACH, we won't acquire the freeze rwsem, but
as TRANS_ATTACH fits the check (trans < TRANS_JOIN_NOLOCK), we'll release the freeze
rwsem, which makes lockdep complains a lot.

Reported-by: Ma Jianpeng <majianpeng@gmail.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>