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 <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>

fs/ext4/inode.c

···
 	 * 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);
 }
 

fs/ext4/mballoc.c

···
 	isize = (i_size_read(ac->ac_inode) + ac->ac_sb->s_blocksize - 1)
 		>> bsbits;
 
-	if ((size == isize) &&
-	    !ext4_fs_is_busy(sbi) &&
-	    (atomic_read(&ac->ac_inode->i_writecount) == 0)) {
+	if ((size == isize) && !ext4_fs_is_busy(sbi) &&
+	    !inode_is_open_for_write(ac->ac_inode)) {
 		ac->ac_flags |= EXT4_MB_HINT_NOPREALLOC;
 		return;
 	}
···
 			(unsigned) ar->goal, ac->ac_flags, ac->ac_2order,
 			(unsigned) ar->lleft, (unsigned) ar->pleft,
 			(unsigned) ar->lright, (unsigned) ar->pright,
-			atomic_read(&ar->inode->i_writecount) ? "" : "non-");
+			inode_is_open_for_write(ar->inode) ? "" : "non-");
 	return 0;
 
 }