vfs: Introduce logging functions

Introduce a set of logging functions through which informational messages,
warnings and error messages incurred by the mount procedure can be logged
and, in a future patch, passed to userspace instead by way of the
filesystem configuration context file descriptor.

There are four functions:

 (1) infof(const char *fmt, ...);

     Logs an informational message.

 (2) warnf(const char *fmt, ...);

     Logs a warning message.

 (3) errorf(const char *fmt, ...);

     Logs an error message.

 (4) invalf(const char *fmt, ...);

     As errof(), but returns -EINVAL so can be used on a return statement.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>