ubifs: fix slab-out-of-bounds in ubifs_change_lp

Hulk Robot reported a KASAN report about slab-out-of-bounds:
 ==================================================================
 BUG: KASAN: slab-out-of-bounds in ubifs_change_lp+0x3a9/0x1390 [ubifs]
 Read of size 8 at addr ffff888101c961f8 by task fsstress/1068
 [...]
 Call Trace:
  check_memory_region+0x1c1/0x1e0
  ubifs_change_lp+0x3a9/0x1390 [ubifs]
  ubifs_change_one_lp+0x170/0x220 [ubifs]
  ubifs_garbage_collect+0x7f9/0xda0 [ubifs]
  ubifs_budget_space+0xfe4/0x1bd0 [ubifs]
  ubifs_write_begin+0x528/0x10c0 [ubifs]

 Allocated by task 1068:
  kmemdup+0x25/0x50
  ubifs_lpt_lookup_dirty+0x372/0xb00 [ubifs]
  ubifs_update_one_lp+0x46/0x260 [ubifs]
  ubifs_tnc_end_commit+0x98b/0x1720 [ubifs]
  do_commit+0x6cb/0x1950 [ubifs]
  ubifs_run_commit+0x15a/0x2b0 [ubifs]
  ubifs_budget_space+0x1061/0x1bd0 [ubifs]
  ubifs_write_begin+0x528/0x10c0 [ubifs]
 [...]
 ==================================================================

In ubifs_garbage_collect(), if ubifs_find_dirty_leb returns an error,
lp is an uninitialized variable. But lp.num might be used in the out
branch, which is a random value. If the value is -1 or another value
that can pass the check, soob may occur in the ubifs_change_lp() in
the following procedure.

To solve this problem, we initialize lp.lnum to -1, and then initialize
it correctly in ubifs_find_dirty_leb, which is not equal to -1, and
ubifs_return_leb is executed only when lp.lnum != -1.

if find a retained or indexing LEB and continue to next loop, but break
before find another LEB, the "taken" flag of this LEB will be cleaned
in ubi_return_lebi(). This bug has also been fixed in this patch.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>