NVMe: Correct sg list setup in nvme_map_user_pages

Our SG list was constructed to always fill the entire first page, even
if that was more than the length of the I/O.  This is probably harmless,
but some IOMMUs might do something bad.

Correcting the first call to sg_set_page() made it look a lot closer to
the sg_set_page() in the loop, so fold the first call to sg_set_page()
into the loop.

Reported-by: Nisheeth Bhat <nisheeth.bhat@intel.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>