fs: fix kernel-doc for dcache::prepend_path

Fix function kernel-doc warning for prepend_path():

Warning(fs/dcache.c:1924): missing initial short description on line:

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>