f2fs: be aware of extent beyond EOF in fiemap

f2fs can support fallocating blocks beyond file size without changing the
size, but ->fiemap of f2fs was restricted and can't detect these extents
fallocated past EOF, now relieve the restriction.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>