dax: Fix deadlock in dax_lock_mapping_entry()

When dax_lock_mapping_entry() has to sleep to obtain entry lock, it will
fail to unlock mapping->i_pages spinlock and thus immediately deadlock
against itself when retrying to grab the entry lock again. Fix the
problem by unlocking mapping->i_pages before retrying.

Fixes: c2a7d2a11552 ("filesystem-dax: Introduce dax_lock_mapping_entry()")
Reported-by: Barret Rhoden <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

fs/dax.c

···
 			xa_unlock_irq(&mapping->i_pages);
 			break;
 		} else if (IS_ERR(entry)) {
+			xa_unlock_irq(&mapping->i_pages);
 			WARN_ON_ONCE(PTR_ERR(entry) != -EAGAIN);
 			continue;
 		}