mtdchar: use kvmalloc() for potentially large allocations

mtdchar_write_ioctl() calls kmalloc() with the 'size' argument set to
the smaller of two values: the write request's data/OOB length provided
by user space and the erase block size of the MTD device.  If the latter
is large, kmalloc() may not be able to serve such allocation requests.
Use kvmalloc() instead.  Correspondingly, replace kfree() calls with
kvfree() calls.

Suggested-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220516070601.11428-3-kernel@kempniu.pl