virtiofsd: check input buffer size in fuse_lowlevel.c ops

virtiofsd: check input buffer size in fuse_lowlevel.c ops

Each FUSE operation involves parsing the input buffer.  Currently the
code assumes the input buffer is large enough for the expected
arguments.  This patch uses fuse_mbuf_iter to check the size.

Most operations are simple to convert.  Some are more complicated due to
variable-length inputs or different sizes depending on the protocol
version.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>