mtd: Fix issue where write_cached_data() fails but write() still returns success

The following sequence is problematic:

mtdblock_flush()
    -->write_cached_data()
        --->erase_write()
        mtdblock: erase of region [0x40000, 0x20000] on "xxx" failed

Problem is: mtdblock_flush() always returns 0. Indeed, even if
write_cached_data() fails and data is not written to the device,
syscall_write() still returns success. Avoid this situation by
actually returning the error coming out of write_cached_data().

Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1584674111-101462-1-git-send-email-nixiaoming@huawei.com