ext4: do not create EA inode under buffer lock

In the Linux kernel, the following vulnerability has been resolved:

ext4: do not create EA inode under buffer lock

ext4_xattr_set_entry() creates new EA inodes while holding buffer lock

on the external xattr block. This is problematic as it nests all the

allocation locking (which acquires locks on other buffers) under the

buffer lock. This can even deadlock when the filesystem is corrupted and

e.g. quota file is setup to contain xattr block as data block. Move the

allocation of EA inode out of ext4_xattr_set_entry() into the callers.