udf: Perform preallocation only for regular files

So far we preallocated blocks also for directories but that brings a
problem, when to get rid of preallocated blocks we don't need. So far
we removed them in udf_clear_inode() which has a disadvantage that
1) blocks are unavailable long after writing to a directory finished
   and thus one can get out of space unnecessarily early
2) releasing blocks from udf_clear_inode is problematic because VFS
   does not expect us to redirty inode there and it also slows down
   memory reclaim.

So preallocate blocks only for regular files where we can drop preallocation
in udf_release_file.

Signed-off-by: Jan Kara <jack@suse.cz>