bpf: Fix a warning message in mark_ptr_not_null_reg()

The WARN_ON() argument is a condition, not an error message.  So this
code will print a stack trace but will not print the warning message.
Fix that and also change it to only WARN_ONCE().

Fixes: 4ddb7416 ("bpf: Extract nullable reg type conversion into a helper function")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/YCzJlV3hnF%2Ft1Pk4@mwanda