ext4: drop ext4_kvmalloc()

As Jan pointed out[1], as of commit 81378da6 ("jbd2: mark the
transaction context with the scope GFP_NOFS context") we use
memalloc_nofs_{save,restore}() while a jbd2 handle is active.  So
ext4_kvmalloc() so we can call allocate using GFP_NOFS is no longer
necessary.

[1] https://lore.kernel.org/r/20200109100007.GC27035@quack2.suse.cz



Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20200116155031.266620-1-tytso@mit.edu


Reviewed-by: Jan Kara <jack@suse.cz>