ffs_mount(): fix snapshotting

Commit 0455cc7104ec broke snapshotting for ffs. In that commit,
ffs_mount() was changed so the namei() lookup for a disk device happens
before ffs_snapshot(). This caused the issue where namei() would lookup
the snapshot file and fail because the file doesn't exist. Even if it did
exist, taking a snapshot would still fail since it's not a disk device.

Fix this by taking a snapshot of the filesystem as-is and return without
altering ro/rw or any other attributes that are passed in.

Reported by: pho
Reviewed by: mckusick
Fixes: 0455cc7104ec ("ffs_mount(): return early if namei() fails to lookup disk device")
Differential Revision: https://reviews.freebsd.org/D34562