os/filestore: copy_range may return OK if fail to verify src/dst offset
os/filestore: fix result code if sanity check failed during copy_range
According to Linux Man Page, the lseek() system call returns -1 and
sets errno only if it indeed encouters some error.
The problem here is if lseek64() returns a positive answer, which
is not strictly equal to the expected answer however, then errno
may not be set and we return success instead, which is wrong.
This commit solves the above problem by checking against the returned
value of lseek64(), if -1 then we reset result code to errno, otherwise
we reset result code to -EINVAL to indicate that it is sanity check not
passed is the real relevant issue.
One more thing, as _do_copy_range() may return customized error code,
caller shall not redirect result code to errno on failure, which is
also shall be considered as wrong action.