struct filename: saner handling of long names

struct filename: saner handling of long names

Always allocate struct filename from names_cachep, long name or short;
short names would be embedded into struct filename.  Longer ones do
not cannibalize the original struct filename - put them into kmalloc'ed
buffers (PATH_MAX-sized for import from userland, strlen() + 1 - for
ones originating kernel-side, where we know the length beforehand).

Cutoff length for short names is chosen so that struct filename would be
192 bytes long - that's both a multiple of 64 and large enough to cover
the majority of real-world uses.

Simplifies logics in getname()/putname() and friends.

[fixed an embarrassing braino in EMBEDDED_NAME_MAX, first reported by
Dan Carpenter]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>