fs/jfs: consolidate sanity checking in dbMount

[ Upstream commit 0d250b1c52484d489e31df2cf9118b7c4bd49d31 ]

Sanity checks have been added to dbMount as individual if clauses with
identical error handling. Move these all into one clause.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Stable-dep-of: 37bfb464ddca ("jfs: validate AG parameters in dbMount() to prevent crashes")
Signed-off-by: Sasha Levin <sashal@kernel.org>