MM/POSIX: check if there is enough space for the posix backing file.
MM/POSIX: check if there is enough space for the posix backing file.

solves #392

sergsagal1/ucx_rocm,sergsagal1/ucx_rocm,shssf/ucx,manjugv/ucx,abouteiller/ucx,abouteiller/ucx,manjugv/ucx,sergsagal1/ucx_rocm,shssf/ucx,abouteiller/ucx,shssf/ucx,manjugv/ucx,abouteiller/ucx