ext4: fold ext4_claim_inode into ext4_new_inode

The function ext4_claim_inode() is only called by one function,
ext4_new_inode(), and by folding the functionality into
ext4_new_inode(), we can remove almost 50 lines of code, and put all
of the logic of allocating a new inode into a single place.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>