erofs: harden h_shared_count in erofs_init_inode_xattrs()

`u8 h_shared_count` indicates the shared xattr count of an inode. It is
read from the on-disk xattr ibody header, which should be corrupted if
the size of the shared xattr array exceeds the space available in
`xattr_isize`.

It does not cause harmful consequence (e.g. crashes), since the image is
already considered corrupted, it indeed results in the silent processing
of garbage metadata.

Let's harden it to report -EFSCORRUPTED earlier.

Signed-off-by: Utkal Singh <singhutkal015@gmail.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>