nvme: Fix potential sign extension issue in nvme_blk_rw()

"lbas" with type "u16" (16 bits, unsigned) is promoted in
"lbas << ns->lba_shift" to type "int" (32 bits, signed), then
sign-extended to type "unsigned long long" (64 bits, unsigned).
If "lbas << ns->lba_shift" is greater than 0x7FFFFFFF, the upper
bits of the result will all be 1.

Fix it by casting "lbas" to "u32".

Reported-by: Coverity (CID: 166730)
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>