eth: fbnic: Update fbnic_tlv_attr_get_string() to work like nla_strscpy()

eth: fbnic: Update fbnic_tlv_attr_get_string() to work like nla_strscpy()

Allow fbnic_tlv_attr_get_string() to return an error code. In the event the
source mailbox attribute is missing return -EINVAL. Like nla_strscpy() return
-E2BIG when the source string is larger than the destination string. In this
case the amount of data copied is equal to dstsize.

Signed-off-by: Lee Trager <lee@trager.us>
Link: https://patch.msgid.link/20250228191935.3953712-3-lee@trager.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>