staging: erofs: fix illegal address access under memory pressure

Considering a read request with two decompressed file pages,
If a decompression work cannot be started on the previous page
due to memory pressure but in-memory LTP map lookup is done,
builder->work should be still NULL.

Moreover, if the current page also belongs to the same map,
it won't try to start the decompression work again and then
run into trouble.

This patch aims to solve the above issue only with little changes
as much as possible in order to make the fix backport easier.

kernel message is:
<4>[1051408f.015930s]SLUB: Unable to allocate memory on node -1, gfp=0x2408040(GFP_NOFS|__GFP_ZERO)
<4>[1051408f.015930s]  cache: erofs_compress, object size: 144, buffer size: 144, default order: 0, min order: 0
<4>[1051408f.015930s]  node 0: slabs: 98, objs: 2744, free: 0
  * Cannot allocate the decompression work

<3>[1051408f.015960s]erofs: z_erofs_vle_normalaccess_readpages, readahead error at page 1008 of nid 5391488
  * Note that the previous page was failed to read

<0>[1051408f.015960s]Internal error: Accessing user space memory outside uaccess.h routines: 96000005 [#1] PREEMPT SMP
...
<4>[1051408f.015991s]Hardware name: kirin710 (DT)
...
<4>[1051408f.016021s]PC is at z_erofs_vle_work_add_page+0xa0/0x17c
<4>[1051408f.016021s]LR is at z_erofs_do_read_page+0x12c/0xcf0
...
<4>[1051408f.018096s][<ffffff80c6fb0fd4>] z_erofs_vle_work_add_page+0xa0/0x17c
<4>[1051408f.018096s][<ffffff80c6fb3814>] z_erofs_vle_normalaccess_readpages+0x1a0/0x37c
<4>[1051408f.018096s][<ffffff80c6d670b8>] read_pages+0x70/0x190
<4>[1051408f.018127s][<ffffff80c6d6736c>] __do_page_cache_readahead+0x194/0x1a8
<4>[1051408f.018127s][<ffffff80c6d59318>] filemap_fault+0x398/0x684
<4>[1051408f.018127s][<ffffff80c6d8a9e0>] __do_fault+0x8c/0x138
<4>[1051408f.018127s][<ffffff80c6d8f90c>] handle_pte_fault+0x730/0xb7c
<4>[1051408f.018127s][<ffffff80c6d8fe04>] __handle_mm_fault+0xac/0xf4
<4>[1051408f.018157s][<ffffff80c6d8fec8>] handle_mm_fault+0x7c/0x118
<4>[1051408f.018157s][<ffffff80c8c52998>] do_page_fault+0x354/0x474
<4>[1051408f.018157s][<ffffff80c8c52af8>] do_translation_fault+0x40/0x48
<4>[1051408f.018157s][<ffffff80c6c002f4>] do_mem_abort+0x80/0x100
<4>[1051408f.018310s]---[ end trace 9f4009a3283bd78b ]---

Fixes: 3883a79a ("staging: erofs: introduce VLE decompression support")
Cc: <stable@vger.kernel.org> # 4.19+
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>