ext2: fix sleeping in atomic bugs on error

The ext2_error() function syncs the filesystem so it sleeps.  The caller
is holding a spinlock so it's not allowed to sleep.

   ext2_statfs() <- disables preempt
   -> ext2_count_free_blocks()
      -> ext2_get_group_desc()

Fix this by using WARN() to print an error message and a stack trace
instead of using ext2_error().

Link: https://lore.kernel.org/r/20210921203233.GA16529@kili


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>