f2fs: fix to update last i_size if fallocate partially succeeds

In the case of expanding pinned file, map.m_lblk and map.m_len
will update in each round of section allocation, so in error
path, last i_size will be calculated with wrong m_lblk and m_len,
fix it.

Fixes: f5a53edc ("f2fs: support aligned pinned file")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>