fs: introduce __getname_gfp()

The purpose of this change is to allow __getname() users to pass a
custom GFP mask to kmem_cache_alloc(). This is needed for annotating
a certain kmemcheck false positive.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>