jffs2: check jffs2_prealloc_raw_node_refs() result in few other places

Fuzzing hit another invalid pointer dereference due to the lack of
checking whether jffs2_prealloc_raw_node_refs() completed successfully.
Subsequent logic implies that the node refs have been allocated.

Handle that. The code is ready for propagating the error upwards.

KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 1 PID: 5835 Comm: syz-executor145 Not tainted 5.10.234-syzkaller #0
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
RIP: 0010:jffs2_link_node_ref+0xac/0x690 fs/jffs2/nodelist.c:600
Call Trace:
 jffs2_mark_erased_block fs/jffs2/erase.c:460 [inline]
 jffs2_erase_pending_blocks+0x688/0x1860 fs/jffs2/erase.c:118
 jffs2_garbage_collect_pass+0x638/0x1a00 fs/jffs2/gc.c:253
 jffs2_reserve_space+0x3f4/0xad0 fs/jffs2/nodemgmt.c:167
 jffs2_write_inode_range+0x246/0xb50 fs/jffs2/write.c:362
 jffs2_write_end+0x712/0x1110 fs/jffs2/file.c:302
 generic_perform_write+0x2c2/0x500 mm/filemap.c:3347
 __generic_file_write_iter+0x252/0x610 mm/filemap.c:3465
 generic_file_write_iter+0xdb/0x230 mm/filemap.c:3497
 call_write_iter include/linux/fs.h:2039 [inline]
 do_iter_readv_writev+0x46d/0x750 fs/read_write.c:740
 do_iter_write+0x18c/0x710 fs/read_write.c:866
 vfs_writev+0x1db/0x6a0 fs/read_write.c:939
 do_pwritev fs/read_write.c:1036 [inline]
 __do_sys_pwritev fs/read_write.c:1083 [inline]
 __se_sys_pwritev fs/read_write.c:1078 [inline]
 __x64_sys_pwritev+0x235/0x310 fs/read_write.c:1078
 do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x67/0xd1

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 2f785402 ("[JFFS2] Reduce visibility of raw_node_ref to upper layers of JFFS2 code.")
Fixes: f560928b ("[JFFS2] Allocate node_ref for wasted space when skipping to page boundary")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>