ext4: use ext4_reserve_inode_write in ext4_xattr_set_handle

ext4_mark_iloc_dirty() says:

 * The caller must have previously called ext4_reserve_inode_write().
 * Give this, we know that the caller already has write access to iloc->bh.

ext4_xattr_set_handle, however, just open-codes it.  May as well use
the helper function for consistency.

No bug here, just tidiness.

(Note: on cleanup path, ext4_reserve_inode_write sets
the bh to NULL if it returns an error, and brelse() of 
a null bh is handled gracefully).

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>