Increase SHM memory to acomodate MEMORY_TARGET

When setting memory_target for oracle you can get errors

-bash-4.1# umount tmpfs
umount: /dev/shm: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

-bash-4.1# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/vda1              28G   19G  7.5G  72% /
tmpfs                 939M  200M  740M  22% /dev/shm

-bash-4.1# umount tmpfs
umount: /dev/shm: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

-bash-4.1# umount -l tmpfs
-bash-4.1# mount -t tmpfs shmfs -o size=2048m /dev/shm
-bash-4.1# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/vda1              28G   19G  7.5G  72% /
shmfs                 2.0G     0  2.0G   0% /dev/shm
-bash-4.1# vi /etc/fstab
tmpfs                   /dev/shm                tmpfs   size=2048m        0 0