ext4: prevent race while walking extent tree for fiemap

Currently ext4_ext_walk_space() only takes i_data_sem for read when
searching for the extent at given block with ext4_ext_find_extent().
Then it drops the lock and the extent tree can be changed at will.
However later on we're searching for the 'next' extent, but the extent
tree might already have changed, so the information might not be
accurate.

In fact we can hit BUG_ON(end <= start) if the extent got inserted into
the tree after the one we found and before the block we were searching
for. This has been reproduced by running xfstests 225 in loop on s390x
architecture, but theoretically we could hit this on any other
architecture as well, but probably not as often.

Moreover the extent currently in delayed allocation might be allocated
after we search the extent tree and before we search extent status tree
delayed buffers resulting in those delayed buffers being completely
missed, even though completely written and allocated.

We fix all those problems in several steps:

 1. remove unnecessary callback indirection
 2. rename functions
        ext4_ext_walk_space -> ext4_fill_fiemap_extents
        ext4_ext_fiemap_cb -> ext4_find_delayed_extent
 3. move fiemap_fill_next_extent() into ext4_fill_fiemap_extents()
 4. hold the i_data_sem for:
        ext4_ext_find_extent()
        ext4_ext_next_allocated_block()
        ext4_find_delayed_extent()
 5. call fiemap_fill_next_extent after releasing the i_data_sem
 6. move path reinitialization into the critical section.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>