ext4: optimize ext4_ext_precache for 0 depth

This patch avoids the memory alloc & free path when depth is 0,
since anyway there is no extra caching done in that case.
So on checking depth 0, simply return early.

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/93da0d0f073c73358e85bb9849d8a5378d1da539.1582880246.git.riteshh@linux.ibm.com


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