[PATCH] document i_size_write locking rules

Unless someone reads the documentation for write_seqcount_{begin,end} it is
not obvious, that i_size_write() needs locking.  Especially, that lack of such
locking can result in a system hang.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>