mtd: check return values of lseek() and write() in mtd_write_buffer()

mtd: check return values of lseek() and write() in mtd_write_buffer()

Two ignored return values in mtd_write_buffer() caused silent failures:

- lseek() return value was ignored. A failed seek (e.g. EBADF, ESPIPE)
  followed by write() would silently write at the wrong offset.
- write() return value was ignored, silently discarding write errors.
  This could lead to data corruption on the MTD device without any
  indication.

Check both return values, report errors to stderr and return -1 on
failure.

Signed-off-by: Anna Kiri <bredcorn@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23552
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>