Linux 3.1 compat, vfs_fsync()

Linux 3.1 compat, vfs_fsync()

Preferentially use the vfs_fsync() function.  This function was
initially introduced in 2.6.29 and took three arguments.  As
of 2.6.35 the dentry argument was dropped from the function.
For older kernels fall back to using file_fsync() which also
took three arguments including the dentry.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #52