rv-virt/citest: Increase init task stack size to 3072

After #15075, the size of the
stack size has decreased 8 bytes and the init stack size for the
rv-virt:citest defconfig was near its full capacity, which lead to
crashing the `ps` command. The init stack size for this defconfig
was increased from 2048 to 3072 to avoid stack overflow.

boards/risc-v/qemu-rv/rv-virt/configs/citest/defconfig

@@ -70,6 +70,7 @@ CONFIG_FS_TMPFS=y
 CONFIG_HAVE_CXX=y
 CONFIG_IDLETHREAD_STACKSIZE=2048
 CONFIG_INIT_ENTRYPOINT="nsh_main"
+CONFIG_INIT_STACKSIZE=3072
 CONFIG_INTELHEX_BINARY=y
 CONFIG_IOB_NBUFFERS=128
 CONFIG_IOB_NCHAINS=4