firmware: qcom: scm: Fix missing read barrier in qcom_scm_get_tzmem_pool()

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

firmware: qcom: scm: Fix missing read barrier in qcom_scm_get_tzmem_pool()

Commit 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq

completion variable initialization") introduced a write barrier in probe

function to store global '__scm' variable. We all known barriers are

paired (see memory-barriers.txt: "Note that write barriers should

normally be paired with read or address-dependency barriers"), therefore

accessing it from concurrent contexts requires read barrier. Previous

commit added such barrier in qcom_scm_is_available(), so let's use that

directly.

Lack of this read barrier can result in fetching stale '__scm' variable

value, NULL, and dereferencing it.

Note that barrier in qcom_scm_is_available() satisfies here the control

dependency.