What's happening is this: Since umsdosfs support is compiled into the kernel(not as a module), when the kernel finishes loading and goes to look for the / mount it first checks for a FAT partition with a folder called linux, with a file called --linux-.--- . If it finds that it will try to mount the file system inside that folder.
Since your system is not umsdos, then it reports the failure to find a 'pseudo-root'.
Also, if you try to mount a partition without specifying the fs type, linux will go through the list of filesystems in /proc/filesystems, and try to mount using each fs type until it is successful. VFAT comes before the linux fs types, so you may get similar errors if you mount without giving fs type or have an 'auto' entry in fs type in fstab.
You can compile a new kernel with no umsdos support (or compiled as a module) to eliminate the 'error' in the kernel messages.
the word 'error' in the kernel messages shouldn't always be taken too seriously.
it's just that the kernel is not too verbose. if the system boots succesfully, most errors can be safely ignored.