drivers: char: mem: use IS_ERR_VALUE() in memory_lseek()

Use IS_ERR_VALUE() instead of comparing the new offset to a hard-coded
value of -MAX_ERRNO (which is also off-by-one due to the use of ~
instead of -).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>