nvme: fix nvme get-feature with -H option

nvme: fix nvme get-feature with -H option

Currently, nvme get-feature with the -H option fails to print detailed
info for specific feature ids such as number of IO queues (fid=0x07),
async event config (fid=0x0b), KATO value (fid=0x0f), etc. This is
because nvme_feature_show_fields() is invoked only if the corresponding
'buf' pointer is valid, which is not necessary for the above feature
ids. So fix this by removing this restriction while invoking
nvme_feature_show_fields(). At the same time, ensure this 'buf' pointer
is appropriately validated before use in this function.

Fixes: bcab212 ("nvme: coverity fixes")
Signed-off-by: Martin George <marting@netapp.com>