in posix_fadvise, don't bypass __syscall macro infrastructure

in posix_fadvise, don't bypass __syscall macro infrastructure

when commit 0b6eb2dfb2e84a8a51906e7634f3d5edc230b058 added the
parentheses around __syscall to invoke the function directly, there
was no __syscall7 in the syscall macro infrastructure, so this hack
was needed. commit 9a3bbce447403d735282586786dc436ec1ffbad4 fixed that
but failed to remove the hack.