ext4: replace opencoded i_writecount usage with inode_is_open_for_write()

There is a function which clearly conveys the objective of checking
i_writecount. Additionally the usage in ext4_mb_initialize_context was
wrong, since a node would have wrongfully been reported as writable if
i_writecount had a negative value (MMAP_DENY_WRITE).

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/inode.c

@@ -391,7 +391,7 @@ void ext4_da_update_reserve_space(struct inode *inode,
 	 * inode's preallocations.
 	 */
 	if ((ei->i_reserved_data_blocks == 0) &&
-	    (atomic_read(&inode->i_writecount) == 0))
+	    !inode_is_open_for_write(inode))
 		ext4_discard_preallocations(inode);
 }