ext4: Introduce ext4_clamp_want_extra_isize()

Based on commit 7bc04c5c "ext4: fix use-after-free race with
debug_want_extra_isize".  We don't have that bug but this will make it
easier to backport commit 4ea99936 "ext4: add more paranoia
checking in ext4_expand_extra_isize handling".

Cc: Barret Rhoden <brho@google.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>