autofs4: use autofs instead of autofs4 everywhere

Update naming within autofs source to be consistent by changing
occurrences of autofs4 to autofs.

Link: http://lkml.kernel.org/r/152626703688.28589.8315406711135226803.stgit@pluto.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>