scsi_bdev: increase max I/O size to 4 MiB

This is necessary to avoid failing very large write_zeroes operations.

This is the minimal workaround to get the current bdevio tests to pass -
the real fix will be implemented in the bdev layer later.

Change-Id: I4e49f1b4da5d4c7f9507757d6c71ba3880d22437
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/383306


Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>