Fix `make distclean` for `--with-config=user`

Fix `make distclean` for `--with-config=user`

Apply the same fix to SPL that was applied to ZFS earlier at:
zfsonlinux/zfs@d433c206515e567c52ce09589033405a0ae3716e

Additionally quote @LINUX_SYMBOLS@ because it is a null substitution
in this configuration, which results in a `[ -f  ]` expression that
incorrectly evaluates to true.

  # ./configure --with-config=user
  # make distclean

  Making distclean in module
  make[1]: Entering directory `/spl/module'
  make -C  SUBDIRS=`pwd`  clean
  make: Entering an unknown directory
  make: *** SUBDIRS=/spl/module: No such file or directory.  Stop.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>