inode_lock_shared() -> down_read(&inode->i_rwsem)
inode_lock() -> down_write(&inode->i_rwsem)
Inode is not updated in f2fs_seek_block(), so there is no need
to hold write lock, use read lock for more efficiency.
Signed-off-by: zhangxirui <xirui.zhang@xxxxxxxx>