baselib: fix unused return value warning

Ignore return value from __copy_from_user_inatomic(). It's okay when it
returns -EFAULT, ktap_lib_user_string() will just leave the string empty
in that case.

Note that this warning only found in x86_64, since __must_check added for
__copy_from_user_inatomic function, but x86_32 doesn't have this attribute.

Signed-off-by: Ben Noordhuis <info@bnoordhuis.nl>