Handle zap_add() failures in mixed case mode

Handle zap_add() failures in mixed case mode

With "casesensitivity=mixed", zap_add() could fail when the number of
files/directories with the same name (varying in case) exceed the
capacity of the leaf node of a Fatzap. This results in a ASSERT()
failure as zfs_link_create() does not expect zap_add() to fail. The fix
is to handle these failures and rollback the transactions.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Chunwei Chen <david.chen@nutanix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Sanjeev Bagewadi <sanjeev.bagewadi@gmail.com>
Closes #7011
Closes #7054