mtd: parser: cmdline: Fix parsing of part-names with colons

mtd: parser: cmdline: Fix parsing of part-names with colons

commit 639a82434f16a6df0ce0e7c8595976f1293940fd upstream.

Some devices (especially QCA ones) are already using hardcoded partition
names with colons in it. The OpenMesh A62 for example provides following
mtd relevant information via cmdline:

  root=31:11 mtdparts=spi0.0:256k(0:SBL1),128k(0:MIBIB),384k(0:QSEE),64k(0:CDT),64k(0:DDRPARAMS),64k(0:APPSBLENV),512k(0:APPSBL),64k(0:ART),64k(custom),64k(0:KEYS),0x002b0000(kernel),0x00c80000(rootfs),15552k(inactive) rootfsname=rootfs rootwait

The change to split only on the last colon between mtd-id and partitions
will cause newpart to see following string for the first partition:

  KEYS),0x002b0000(kernel),0x00c80000(rootfs),15552k(inactive)

Such a partition list cannot be parsed and thus the device fails to boot.

Avoid this behavior by making sure that the start of the first part-name
("(") will also be the last byte the mtd-id split algorithm is using for
its colon search.

Fixes: eb13fa022741 ("mtd: parser: cmdline: Support MTD names containing one or more colons")
Cc: stable@vger.kernel.org
Cc: Ron Minnich <rminnich@google.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201124062506.185392-1-sven@narfation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>