f2fs: remove checkpoint in f2fs_freeze

f2fs: remove checkpoint in f2fs_freeze

The generic freeze_super() calls sync_filesystems() before f2fs_freeze().
So, basically we don't need to do checkpoint in f2fs_freeze(). But, in xfs/068,
it triggers circular locking problem below due to gc_mutex for checkpoint.

======================================================
[ INFO: possible circular locking dependency detected ]
4.9.0-rc1+ #132 Tainted: G           OE
-------------------------------------------------------

1. wait for __sb_start_write() by

 [<ffffffff9845f353>] dump_stack+0x85/0xc2
 [<ffffffff980e80bf>] print_circular_bug+0x1cf/0x230
 [<ffffffff980eb4d0>] __lock_acquire+0x19e0/0x1bc0
 [<ffffffff980ebdcb>] lock_acquire+0x11b/0x220
 [<ffffffffc08c7c3b>] ? f2fs_drop_inode+0x9b/0x160 [f2fs]
 [<ffffffff9826bdd0>] __sb_start_write+0x130/0x200
 [<ffffffffc08c7c3b>] ? f2fs_drop_inode+0x9b/0x160 [f2fs]
 [<ffffffffc08c7c3b>] f2fs_drop_inode+0x9b/0x160 [f2fs]
 [<ffffffff98289991>] iput+0x171/0x2c0
 [<ffffffffc08cfccf>] f2fs_sync_inode_meta+0x3f/0xf0 [f2fs]
 [<ffffffffc08cfe04>] block_operations+0x84/0x110 [f2fs]
 [<ffffffffc08cff78>] write_checkpoint+0xe8/0xf20 [f2fs]
 [<ffffffff980e979d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffffc08c6de9>] ? f2fs_sync_fs+0x79/0x190 [f2fs]
 [<ffffffff9803e9d9>] ? sched_clock+0x9/0x10
 [<ffffffffc08c6de9>] ? f2fs_sync_fs+0x79/0x190 [f2fs]
 [<ffffffffc08c6df5>] f2fs_sync_fs+0x85/0x190 [f2fs]
 [<ffffffff982a4f90>] ? do_fsync+0x70/0x70
 [<ffffffff982a4f90>] ? do_fsync+0x70/0x70
 [<ffffffff982a4fb0>] sync_fs_one_sb+0x20/0x30
 [<ffffffff9826ca3e>] iterate_supers+0xae/0x100
 [<ffffffff982a50b5>] sys_sync+0x55/0x90
 [<ffffffff9890b345>] entry_SYSCALL_64_fastpath+0x23/0xc6

2. wait for sbi->gc_mutex by

 [<ffffffff980ebdcb>] lock_acquire+0x11b/0x220
 [<ffffffff989063d6>] mutex_lock_nested+0x76/0x3f0
 [<ffffffffc08c6de9>] f2fs_sync_fs+0x79/0x190 [f2fs]
 [<ffffffffc08c7a6c>] f2fs_freeze+0x1c/0x20 [f2fs]
 [<ffffffff9826b6ef>] freeze_super+0xcf/0x190
 [<ffffffff9827eebc>] do_vfs_ioctl+0x53c/0x6a0
 [<ffffffff9827f099>] SyS_ioctl+0x79/0x90
 [<ffffffff9890b345>] entry_SYSCALL_64_fastpath+0x23/0xc6

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>