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

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>