linux: improve reporting for unsupported syscall flags
Filter out the flags we do support; previously we would print
out the flag value verbatim.
Reviewed By: dchagin
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30693
(cherry picked from commit 9d167945e8409c739442a02e3fbacb3bcdcc3306)