loop: fix error handling regression

gcc points out an unusual indentation:

drivers/block/loop.c: In function 'loop_set_status':
drivers/block/loop.c:1149:3: error: this 'if' clause does not guard... [-Werror=misleading-indentation]
   if (figure_loop_size(lo, info->lo_offset, info->lo_sizelimit,
   ^~
drivers/block/loop.c:1152:4: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
    goto exit;

This was introduced by a new feature that accidentally moved the opening
braces from one condition to another. Adding a second pair of braces
makes it work correctly again and also more readable.

Fixes: f2c6df7d

 ("loop: support 4k physical blocksize")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@fb.com>