Fix fget_only_user() to return ENOTCAPABLE on a failed capsicum check

[arichardson]

Summary

After eaad8d1303da500ed691bd774742a4555a05e729 four additional
capsicum-test tests started failing. It turns out this is because
fget_only_user() was returning EBADF on a failed capsicum check instead
of forwarding the return value of cap_check_inline() like
fget_unlocked_seq().

Test Plan

capsicum-test failures before this:

[  FAILED  ] 7 tests, listed below:
[  FAILED  ] Capability.OperationsForked
[  FAILED  ] Capability.NoBypassDAC
[  FAILED  ] Pdfork.OtherUserForked
[  FAILED  ] PipePdfork.WildcardWait
[  FAILED  ] OpenatTest.WithFlag
[  FAILED  ] ForkedOpenatTest_WithFlagInCapabilityMode._
[  FAILED  ] Select.LotsOFileDescriptorsForked

After:

[  FAILED  ] 3 tests, listed below:
[  FAILED  ] Capability.NoBypassDAC
[  FAILED  ] Pdfork.OtherUserForked
[  FAILED  ] PipePdfork.WildcardWait