fs: fs_mount parameter verification before mutex locked block

The commit moves fs_mount parameter verification above mutex lock.
The list of mount points is now checked before attempting to obtain
file system API pointer.
All modifications to mount point data structure, given as a parameter
to the fs_mount, are only applied after every other operation needed
have completed successfully, immediately before adding the mount point
to the list of mount points.
The fs_mount will a warning when mounted file system does not support
unmount.
When a file system does not provide mount function, the -ENOTSUP error
will be returned instead of -EINVAL.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>