f2fs: file: drop useless initializer in expand_inode_data()

In expand_inode_data(), the 'new_size' local variable is initialized to
the result of i_size_read(), however this value isn't ever used,  so we
can drop this initializer...

Found by Linux Verification Center (linuxtesting.org) with the SVACE static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>