ubifs: Check @c->dirty_[n|p]n_cnt and @c->nroot state under @c->lp_mutex

The checking of @c->nroot->flags and @c->dirty_[n|p]n_cnt in function
nothing_to_commit() is not atomic, which could be raced with modifying
of lpt, for example:
       P1        P2        P3
run_gc
 ubifs_garbage_collect
              do_commit
 ubifs_return_leb
  ubifs_lpt_lookup_dirty
   dirty_cow_nnode
                       do_commit
			nothing_to_commit
			 if (test_bit(DIRTY_CNODE, &c->nroot->flags)
			 // false
   test_and_set_bit(DIRTY_CNODE, &nnode->flags)
   c->dirty_nn_cnt += 1
                         ubifs_assert(c, c->dirty_nn_cnt == 0)
			 // false !

Fetch a reproducer in Link:
 UBIFS error (ubi0:0 pid 2747): ubifs_assert_failed
 UBIFS assert failed: c->dirty_pn_cnt == 0, in fs/ubifs/commit.c
 Call Trace:
  ubifs_ro_mode+0x58/0x70 [ubifs]
  ubifs_assert_failed+0x6a/0x90 [ubifs]
  do_commit+0x5b7/0x930 [ubifs]
  ubifs_run_commit+0xc6/0x1a0 [ubifs]
  ubifs_sync_fs+0xd8/0x110 [ubifs]
  sync_filesystem+0xb4/0x120
  do_syscall_64+0x6f/0x140

Fix it by checking @c->dirty_[n|p]n_cnt and @c->nroot state with
@c->lp_mutex locked.

Fixes: 944fdef5 ("UBIFS: do not start the commit if there is nothing to commit")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218162


Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>