reiserfs: Fix unwanted recursive reiserfs lock in reiserfs_unlink()

reiserfs_unlink() may or may not be called under the reiserfs
lock.
But it also takes the reiserfs lock and can then acquire it
recursively which leads to do_journal_begin_r() that fails to
relax the reiserfs lock before grabbing the journal mutex,
creating an unexpected lock inversion.

We need to ensure reiserfs_unlink() won't get the reiserfs lock
recursively using reiserfs_write_lock_once().

This fixes the following warning that precedes a lock inversion
report (reiserfs lock <-> journal mutex).

------------[ cut here ]------------
WARNING: at fs/reiserfs/lock.c:95 reiserfs_lock_check_recursive+0x3a/0x50()
Hardware name: MS-7418
Unwanted recursive reiserfs lock!
Pid: 3208, comm: dbench Not tainted 2.6.32-atom #177
Call Trace:
 [<c114327a>] ? reiserfs_lock_check_recursive+0x3a/0x50
 [<c114327a>] ? reiserfs_lock_check_recursive+0x3a/0x50
 [<c10373a7>] warn_slowpath_common+0x67/0xc0
 [<c114327a>] ? reiserfs_lock_check_recursive+0x3a/0x50
 [<c1037446>] warn_slowpath_fmt+0x26/0x30
 [<c114327a>] reiserfs_lock_check_recursive+0x3a/0x50
 [<c113c213>] do_journal_begin_r+0x83/0x360
 [<c105eb16>] ? __lock_acquire+0x1296/0x19e0
 [<c1142a57>] ? xattr_unlink+0x57/0xb0
 [<c113c670>] journal_begin+0x80/0x130
 [<c1116d5d>] reiserfs_unlink+0x7d/0x2d0
 [<c1142a57>] ? xattr_unlink+0x57/0xb0
 [<c1142a57>] ? xattr_unlink+0x57/0xb0
 [<c1142a57>] ? xattr_unlink+0x57/0xb0
 [<c1142a64>] xattr_unlink+0x64/0xb0
 [<c1143169>] delete_one_xattr+0x29/0x100
 [<c11427ab>] reiserfs_for_each_xattr+0x10b/0x290
 [<c1143140>] ? delete_one_xattr+0x0/0x100
 [<c1401ca9>] ? mutex_lock_nested+0x299/0x340
 [<c11429aa>] reiserfs_delete_xattrs+0x1a/0x60
 [<c11432f9>] ? reiserfs_write_lock_once+0x29/0x50
 [<c111ea1f>] reiserfs_delete_inode+0x9f/0x150
 [<c11b0d0f>] ? _atomic_dec_and_lock+0x4f/0x70
 [<c111e980>] ? reiserfs_delete_inode+0x0/0x150
 [<c10c9c32>] generic_delete_inode+0xa2/0x170
 [<c10c9d4f>] generic_drop_inode+0x4f/0x70
 [<c10c8b07>] iput+0x47/0x50
 [<c10c0965>] do_unlinkat+0xd5/0x160
 [<c10505c6>] ? up_read+0x16/0x30
 [<c1022ab7>] ? do_page_fault+0x187/0x330
 [<c1002fd8>] ? restore_all_notrace+0x0/0x18
 [<c1022930>] ? do_page_fault+0x0/0x330
 [<c105cbe4>] ? trace_hardirqs_on_caller+0x124/0x170
 [<c10c0a00>] sys_unlink+0x10/0x20
 [<c1002ec4>] sysenter_do_call+0x12/0x32
---[ end trace 2e35d71a6cc69d0c ]---

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Christian Kujau <lists@nerdbynature.de>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>