btrfs: Perform locking/unlocking in btrfs_remap_file_range()

Move code to make it more readable, so as locking and unlocking is
done in the same function. The generic checks that are now performed in
the locked section are unaffected.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>