RDMA/hfi1: Prevent panic when SDMA is disabled

In the Linux kernel, the following vulnerability has been resolved:

RDMA/hfi1: Prevent panic when SDMA is disabled

If the hfi1 module is loaded with HFI1_CAP_SDMA off, a call to

hfi1_write_iter() will dereference a NULL pointer and panic. A typical

stack frame is:

sdma_select_user_engine [hfi1]

hfi1_user_sdma_process_request [hfi1]

hfi1_write_iter [hfi1]

do_iter_readv_writev

do_iter_write

vfs_writev

do_writev

do_syscall_64

The fix is to test for SDMA in hfi1_write_iter() and fail the I/O with

EINVAL.