fabrics: fix potential invalid memory access in __nvmf_supported_option()

fabrics: fix potential invalid memory access in __nvmf_supported_option()

In __nvmf_supported_option(), len is declared as size_t (unsigned)

"len = read()" may return a negative number;
the check "if (len < 0)" will always be false and therefore
"buf[len]" will dereference an invalid memory address.

len should be declared as a signed size_t (ssize_t)

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>