We're using svc_take_page here to get another page for the tail in case one
wasn't already allocated.  But there isn't always guaranteed to be another
page available.

Also fix a typo that made us check the tail buffer for space when we meant to
be checking the head buffer.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NNeil Brown <neilb@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>