f2fs: fix to initialize map.m_pblk in f2fs_precache_extents()

stable inclusion
from stable-v6.6.2
commit f1aeaf3c47c9d76b309b92a5bd13208ea30c5fb0
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8IW7G

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f1aeaf3c47c9d76b309b92a5bd13208ea30c5fb0



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[ Upstream commit 8b07c1fb0f1ad139373c8253f2fad8bc43fab07d ]

Otherwise, it may print random physical block address in tracepoint
of f2fs_map_blocks() as below:

f2fs_map_blocks: dev = (253,16), ino = 2297, file offset = 0, start blkaddr = 0xa356c421, len = 0x0, flags = 0

Fixes: c4020b2d ("f2fs: support F2FS_IOC_PRECACHE_EXTENTS")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>