Fix fallocate erroneously returning ENOSYS on Linux systems

Fix fallocate erroneously returning ENOSYS on Linux systems

Currently, the helpers.c file provides a function defintion for Linux
fallocate that sets errno to ENOSYS and returns failure; this is useful
for a non-Linux OS. However, this definition will override the Linux
implementation of fallocate when 'fallocate=keep' is set. Adding a
preprocessor macro to check if Linux fallocate is defined fixes this
issue.

Signed-off-by: Wade Cline <wcline@us.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>