IB/mthca: Use bitmap_zalloc() when applicable

Use 'bitmap_zalloc()' to simplify code, improve the semantic and avoid
some open-coded arithmetic in allocator arguments.

Using the 'zalloc' version of the allocator also saves a now useless
'bitmap_zero()' call.

Also change the corresponding 'kfree()' into 'bitmap_free()' to keep
consistency.

Link: https://lore.kernel.org/r/ea9031e28f453bc179033740f66f0c19293fcf0b.1637785902.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>