dax: Fix sleep in atomic contex in grab_mapping_entry()

Commit 642261ac: "dax: add struct iomap based DAX PMD support" has
introduced unmapping of page tables if huge page needs to be split in
grab_mapping_entry(). However the unmapping happens after
radix_tree_preload() call which disables preemption and thus
unmap_mapping_range() tries to acquire i_mmap_lock in atomic context
which is a bug. Fix the problem by moving unmapping before
radix_tree_preload() call.

Fixes: 642261ac
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>