mtd: part: reject MTDPART_OFS_RETAIN in mtd_add_partition()

In the Linux kernel, the following vulnerability has been resolved:

mtd: part: reject MTDPART_OFS_RETAIN in mtd_add_partition()

mtd_add_partition() does not reject the special offset value
MTDPART_OFS_RETAIN (-3), which leads to a WARN_ON in
add_mtd_device() when called through the BLKPG ioctl on NAND
devices. The RETAIN value depends on cur_offset being the end of
the previous partition, but in the dynamic partition path
cur_offset equals the offset argument itself, causing undefined
behavior.

Commit 5daa7b21496a ("mtd: prepare partition add and del functions
for ioctl requests") introduced mtd_add_partition() and correctly
rejected MTDPART_OFS_APPEND (-1) and MTDPART_OFS_NXTBLK (-2),
since those special offsets rely on cur_offset tracking the
previous partition's end. However, commit 1a31368bf92e ("mtd: add a flags
for partitions which should just leave smth. after them")
later added MTDPART_OFS_RETAIN (-3) for the static
partition table path without updating mtd_add_partition() to
also reject this value.

With offset=-3 passed via BLKPG, the RETAIN size calculation in
allocate_partition() underflows (parent_size - 0xFFFFFFFFFFFFFFFD
= parent_size + 3). If the underflow result does not appear to
leave enough space, allocate_partition() jumps to out_register via
goto, skipping erasesize initialization. This results in
erasesize=0, which triggers:

WARN_ON((!mtd->erasesize || !master->_erase) &&
!(mtd->flags & MTD_NO_ERASE))

in add_mtd_device(). If the underflow result appears to leave
enough space, a bogus partition size is calculated, but the
"out of reach" sanity check catches the invalid offset and
creates a disabled empty partition (offset=0, size=0) instead
of returning an error.

Fix this by adding MTDPART_OFS_RETAIN to the rejection list in
mtd_add_partition(), consistent with the existing handling of
APPEND and NXTBLK.