quota: Use the pre-processor to compile out quotactl_cmd_write when !CONFIG_BLOCK

quotactl_cmd_write() is only ever invoked when BLOCK is configured. When
!CONFIG_BLOCK, the build warning below is displayed. Let's fix that.

fs/quota/quota.c:311:12: warning: ‘quotactl_cmd_write’ defined but not used [-Wunused-function]

Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jan Kara <jack@suse.cz>