Skip AllocateTest if fallocate() is not supported in the file system

Summary: To avoid false positive test failures when the file system doesn't support fallocate. In EnvTest.AllocateTest, we first make a simple fallocate call and check the error codes to rule out the possibility that it is not supported. Skip the test if the error code indicates it is not supported.

Test Plan: Run the test and make sure it passes on file systems supporting and not supporting fallocate

Reviewers: yhchiang, ljin, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23667