Fix error code handling for failed cap_reset_ambient()

Looks like the system call wrapper wasn't migrated properly when
I added support to get fakeroot (

  https://bugzilla.kernel.org/show_bug.cgi?id=206539

) working again. That is, all builds in the inclusive range
libcap-[2.28, 2.53] have this issue.

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>