ocfs2: protect extent tree in ocfs2_prepare_inode_for_write()

When the extent tree is modified, it should be protected by inode
cluster lock and ip_alloc_sem.

The extent tree is accessed and modified in the
ocfs2_prepare_inode_for_write, but isn't protected by ip_alloc_sem.

The following is a case.  The function ocfs2_fiemap is accessing the
extent tree, which is modified at the same time.

  kernel BUG at fs/ocfs2/extent_map.c:475!
  invalid opcode: 0000 [#1] SMP
  Modules linked in: tun ocfs2 ocfs2_nodemanager configfs ocfs2_stackglue [...]
  CPU: 16 PID: 14047 Comm: o2info Not tainted 4.1.12-124.23.1.el6uek.x86_64 #2
  Hardware name: Oracle Corporation ORACLE SERVER X7-2L/ASM, MB MECH, X7-2L, BIOS 42040600 10/19/2018
  task: ffff88019487e200 ti: ffff88003daa4000 task.ti: ffff88003daa4000
  RIP: ocfs2_get_clusters_nocache.isra.11+0x390/0x550 [ocfs2]
  Call Trace:
    ocfs2_fiemap+0x1e3/0x430 [ocfs2]
    do_vfs_ioctl+0x155/0x510
    SyS_ioctl+0x81/0xa0
    system_call_fastpath+0x18/0xd8
  Code: 18 48 c7 c6 60 7f 65 a0 31 c0 bb e2 ff ff ff 48 8b 4a 40 48 8b 7a 28 48 c7 c2 78 2d 66 a0 e8 38 4f 05 00 e9 28 fe ff ff 0f 1f 00 <0f> 0b 66 0f 1f 44 00 00 bb 86 ff ff ff e9 13 fe ff ff 66 0f 1f
  RIP  ocfs2_get_clusters_nocache.isra.11+0x390/0x550 [ocfs2]
  ---[ end trace c8aa0c8180e869dc ]---
  Kernel panic - not syncing: Fatal exception
  Kernel Offset: disabled

This issue can be reproduced every week in a production environment.

This issue is related to the usage mode.  If others use ocfs2 in this
mode, the kernel will panic frequently.

[akpm@linux-foundation.org: coding style fixes]
[Fix new warning due to unused function by removing said function - Linus ]
Link: http://lkml.kernel.org/r/1568772175-2906-2-git-send-email-sunny.s.zhang@oracle.com


Signed-off-by: Shuning Zhang <sunny.s.zhang@oracle.com>
Reviewed-by: Junxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: Gang He <ghe@suse.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Joseph Qi <jiangqi903@gmail.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>