drm/amdkfd: Fix use-after-free of HMM range in svm_range_validate_and_map()

drm/amdkfd: Fix use-after-free of HMM range in svm_range_validate_and_map()

The function svm_range_validate_and_map() was freeing `range` when
amdgpu_hmm_range_get_pages() failed. But later, the code still used the
same `range` pointer and freed it again. This could cause a
use-after-free and double-free issue.

The fix sets `range = NULL` right after it is freed and checks for
`range` before using or freeing it again.

v2: Removed duplicate !r check in the condition for clarity.

v3: In amdgpu_hmm_range_get_pages(), when hmm_range_fault() fails, we
kvfree(pfns) but leave the pointer in hmm_range->hmm_pfns still pointing
to freed memory. The caller (or amdgpu_hmm_range_free(range)) may try to
free range->hmm_range.hmm_pfns again, causing a double free, Setting
hmm_range->hmm_pfns = NULL immediately after kvfree(pfns) prevents both
double free. (Philip)

In svm_range_validate_and_map(), When r == 0, it means success → range
is not NULL.  When r != 0, it means failure → already made range = NULL.
So checking both (!r && range) is unnecessary because the moment r == 0,
we automatically know range exists and is safe to use. (Philip)

Fixes: 737da5363cc0 ("drm/amdgpu: update the functions to use amdgpu version of hmm")
Reported by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Philip Yang <Philip.Yang@amd.com>
Cc: Sunil Khatri <sunil.khatri@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Philip Yang<Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>