Allow spaces in pool names for cmdline argument

PR #8114 quoted the ${ENCRYPTIONROOT} parameter to ensure we don't
lose spaces when unlocking root filesystem in the off chance that 
it has a space in its name.

Unfortunately, dracut and initramfs-tools do not actually get the 
quotes from the cmdline. If we use root=ZFS="root pool/filesystem 
name" the script still only sees root=ZFS=root and no quotation 
marks.

Because + is a reserved character in ZFS, it's used as a 
placeholder for spaces in the kernel cmdline.  In this way,
root=ZFS=root+pool/filesystem+name will properly expand by 
replacing the character with sed (POSIX compliant method).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Signed-off-by: Kash Pande <kash@tripleback.net>
Issue #8114 
Closes #8117

contrib/dracut/90zfs/parse-zfs.sh.in

@@ -39,11 +39,14 @@ case "${root}" in
 		# root=zfs:rpool/ROOT
 		# root=zfs:FILESYSTEM=rpool/ROOT
 		# root=FILESYSTEM=rpool/ROOT
+		# root=ZFS=pool+with+space/ROOT+WITH+SPACE (translates to root=ZFS=pool with space/ROOT WITH SPACE)
 
 		# Strip down to just the pool/fs
 		root="${root#zfs:}"
 		root="${root#FILESYSTEM=}"
 		root="zfs:${root#ZFS=}"
+		# switch + with spaces because kernel cmdline does not allow us to quote parameters
+		root=$(printf '%s\n' "$root" | sed "s/+/ /g")
 		rootok=1
 		wait_for_zfs=1