sio: fix call to vfs_fsync() for newest rhel6 kernels

the number of arguments to vfs_fsync has changed in kernel 2.6.35.
The S_BIAS macro (removed at about the same time in 2.6.35) is used to
detect whether vfs_fsync must be called with 2 or 3 args. In RHEL6
kernels 2.6.32, the removal of S_BIAS was backported, but the change
in vfs_fsync was not. So a check for RHEL_MAJOR < 7 is used in
addition to S_BIAS to find the correct number of args for vfs_fsync
call.

Signed-off-by: Thomas Schoebel-Theuer <tst@1und1.de>
doc