fuse: limit fusermount fallback to EPERM cases

There are two mount mechanims for fuse:
1) Call mount(2) syscall directly -- implemented by fuse_mount_sys
2) Call out to fusermount(1) helper utilty to do the mount --
   implemented by fuse_mount_fusermount
   [Note: both libfuse and glusterfs ships a variant of this helper
   utility; named, respectively, fusermount and fusermount-glusterfs.
   The two has diverged, and are not compatible at the moment.]

The intended use of 1) is privileged mounting, ie. when root
is invoking the glusterfs client. (It cannot work for non-privileged
users as the kernel enforces privilege for mount(2), more precisely,
caller context needs CAP_SYS_ADMIN, see capabilities(7).)

The intended use of 2) is unprivileged mountig, ie. when
the glusterfs client is invoked by an unprivileged user.
The helper utility is a setuid binary, so it can perform
mount(2) on behalf of the user.

The main mount routine, gf_fuse_mount, calls fuse_mount_sys first,
and if that fails, tries also with fuse_mount_fusermount. This
is what we call "fusermount fallback". However, in the light of
the above remarks about intended use, this logic should apply if
the fuse_mount_fusermount fails because of a privilege shortage,
ie. with error "Operation not permitted" (errno EPERM).

So far the fallback was unconditional (masking bugs of
fuser_mount_sys, as it happens in referred BUG). Now we
add the "errno == EPERM" condition.

BUG: 1297182
Change-Id: Ia89d975d1e27fcfa5ab2036ba546aa8fa0d2d1b0
Signed-off-by: Csaba Henk <csaba@redhat.com>
Reviewed-on: http://review.gluster.org/15766
Smoke: Gluster Build System <jenkins@build.gluster.org>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>

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