f2fs: use inode_lock_shared instead of inode_lock in f2fs_seek_block()

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@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>