[PATCH] _proc_do_string(): fix short reads

If you try to read things like /proc/sys/kernel/osrelease with single-byte
reads, you get just one byte and then EOF.  This is because _proc_do_string()
assumes that the caller is read()ing into a buffer which is large enough to
fit the whole string in a single hit.

Fix.

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>