ext4: generalize extents status tree search functions

Ext4 contains a few functions that are used to search for delayed
extents or blocks in the extents status tree.  Rather than duplicate
code to add new functions to search for extents with different status
values, such as written or a combination of delayed and unwritten,
generalize the existing code to search for caller-specified extents
status values.  Also, move this code into extents_status.c where it
is better associated with the data structures it operates upon, and
where it can be more readily used to implement new extents status tree
functions that might want a broader scope for i_es_lock.

Three missing static specifiers in RFC version of patch reported and
fixed by Fengguang Wu <fengguang.wu@intel.com>.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>