syscall: fixed mincore(2) not failing with ENOMEM

The current implementation of the mincore(2) syscall sets errno to
EFAULT when the region identified by the first two parameters is
invalid.

This goes against the man page specification, where mincore(2) should
only fail with EFAULT when the third parameter is an invalid address;
and fail with ENOMEM when the checked region does not point to mapped
memory.

Signed-off-by: Franklin "Snaipe" Mathieu <snaipe@diacritic.io>
Cc: Riku Voipio <riku.voipio@linaro.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>