It should be pass "newsize" to vmtruncate function to modify the
inode->i_size, while the old size is passed to vmtruncate.

This bug was caught by LTP truncate test case on Blackfin platform.
After it was fixed, the LTP truncate test case passed.
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>