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.