ext4: assign get_fs()->dev_desc before using it

Commit 50ce4c07 "fs/ext4: Support device block sizes != 512 bytes"
modified ext4fs_set_blk_dev() to calculate total_sect based on
get_fs()->dev_desc->log2blksz rather than SECTOR_SIZE. However, this
value wasn't yet assigned. Move the assignment earlier so the code
doesn't crash or hang.

Cc: Egbert Eich <eich@suse.com>
Tested-by: Tom Rini <trini@ti.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>