[scudo] Lock FallbackTSD before draining it Differential Revision: https://reviews.llvm.org/D150301 Bug: 281740882 GitOrigin-RevId: 6561032e1d3c561b3f7ced817bf3350b8622bf92 Change-Id: I4225b1f9a9c4e353553c92627591f1f5695e9227 Mreged-In: I4225b1f9a9c4e353553c92627591f1f5695e9227 (cherry picked from commit c3ba0f1d13d228616f57a714a3fc28a1fae83b37)