btrfs: do not allow mounting non-subvolumes via subvol option

There's a missing test whether the path passed to subvol=path option
during mount is a real subvolume, allowing any directory located in
default subovlume to be passed and accepted for mount.

(current btrfs progs prevent this early)
$ btrfs subvol snapshot . p1-snap
ERROR: '.' is not a subvolume

(with "is subvolume?" test bypassed)
$ btrfs subvol snapshot . p1-snap
Create a snapshot of '.' in './p1-snap'

$ btrfs subvol list -p .
ID 258 parent 5 top level 5 path subvol
ID 259 parent 5 top level 5 path subvol1
ID 260 parent 5 top level 5 path default-subvol1
ID 262 parent 5 top level 5 path p1/p1-snapshot
ID 263 parent 259 top level 5 path subvol1/subvol1-snap

The problem I see is that this makes a false impression of snapshotting the
given subvolume but in fact snapshots the default one: a user expects outcome
like ID 263 but in fact gets ID 262 .

This patch makes mount fail with EINVAL with a message in syslog.

Signed-off-by: David Sterba <dsterba@suse.cz>